Growth Forms

Mullis and Sigl (1982) describe such hollow quartz crystals and aggregates of these quartzes they found in a trachyte from the west coast of Gran Canaria, Spain. The authors speculate that they form in boiling watery solutions and that the crystals grew around bubbles attached to the crystal tips. This view is supported by the finding of crystals whose tips were completed again and in which the cavity was sealed. Such cavities were found to contain water vapor. It is nevertheless not clear if growth inhibition by vapor bubbles is the general mechanism behind the formation of these and similar crystals: some of the hollow crystals get fairly large and have an internal diameter of one centimeter but are just one centimeter high. It is difficult to see how a small crystal "grows its way around" a large bubble - the fastest growth occurs parallel to the c-axis, so a crystal that encloses a large bubble must already have a diameter that is close to that of the bubble.

10mm  1568x2240 340kb - 3136x4480 973kb

This specimen is from the Opal Hill Mine at Palo Verde, Imperial County, California. This spot is one of the few fire agate localities (fire agate is presented in the chalcedony subsection), but you can also find chalcedony roses. In the upper half of the specimen you see a small aggregate of six-sided hollow crystals inside such a chalcedony rose.

10mm  2112x1424 434kb - 4224x2848 1224kb

A close-up of the honeycomb-like crystal aggregate shown in the previous image. This is the only specimen I have where several hexagons form a structure not unlike a small wasp nest. Note that the crystals outer shape is clearly hexagonal like an ordinary quartz crystal prism, but the internal walls are rounded.

10mm  2080x1440 527kb - 4160x2880 1480kb

This image shows a cavity in a volcanic host rock that is outlined by irregularly intergrown crystals. The larger ones assumed a cup-like shape, like the big one in the center of the image with an internal diameter of about 1 cm. On its left side there is another "cup" in which smaller crystals started to grow from the ground of the cup and form multiple terminations. From Tillie Hall Peak, Mule Creek, Graham County, New Mexico.

5mm  1424x1760 415kb - 2848x3520 1167kb

Two hollow crystals on the specimen shown in the previous image (but not visible on that photo). By a lucky coincidence the right crystal has been split parallel to the c-axis so the hollow tube can be seen framed by the shiny fracture surfaces left and right to it. There is a "plug" in the tube with a small crystal tip on top of it (pixel coordinates 1805,1284, counting from the upper left corner at camera resolution) and another one at its bottom (pixel coordinates 1937,2148). The lower end of the cavity is marked by black dots of an unknown mineral (possibly iron or manganese oxides). The left crystal is mostly intact, only at its base (around the pixel coordinates 1316,2032) there is a little hole and one can look into the cavity. In both cases the cavity reaches the base of the crystals, it is not just a shallow dent, and the diameter of the cavity amounts to about 30-60% of the external diameter of the crystal. Note the peculiar surface pattern on its prism faces, resulting from several layers that have been deposited on the crystal.

5mm  1680x1408 294kb - 3360x2816 827kb

This is another specimen from the Opal Hill mine, with a group of hollow crystals radiating from a point in the bottom right corner of the image. The hollow crystal in the center that points to the top left corner shows a scepter-like overgrowth of the prism: a group of three small crystal tips points in the opposite direction. A similar scepter-like growth pattern is present in the neighboring crystals, albeit it is more difficult to see. The scepter-like overgrowth of the prism is also found in crystals from Tillie Hall Peak, the Mopah Range and in the locality on Gran Canaria, so it seems to be typical of such hollow crystals. The crystal above the one in the center is partially overgrown by dull translucent chalcedony, which has also reported for Gran Canaria specimen.

Growth Inhibition and Mechanical Interference

20mm  1464x1024 220kb - 2928x2048 630kb

10mm  976x1378 137kb - 1952x2756 407kb

This smoky quartz has an interesting pattern on the right side of its frontal prism face. There are three dents that neighboring rocks or crystals have left when the entire pocket got deformed during tectonic activity. This happened at the end of the crystal's growth phase and the mechanical stress exerted on those spots caused the formation of a peculiar surface pattern of roughly circular walls around the dents. The crystal is from Peterson Mountain, Sierra County, at the California-Nevada border, a tectonically still very active area.

10mm  1048x1520 149kb - 2096x3039 450kb

This double-terminated clear crystal shows many imprints of crystals that have later been dissolved. Judging from the shape of the indentations, these crystals have very likely been calcite, siderite, or dolomite, which all are carbonates that easily get dissolved upon changes in temperature, acidity or CO₂ concentration. The indentations have later been filled with green chlorite. From an alpine-type fissure close to the Fieschergletscher (Fiescher Glacier), Wallis, Switzerland.

20mm  1760x1568 396kb - 3520x3136 1117kb 10mm  2000x1440 535kb - 4000x2880 1539kb

An  unknown mineral, possibly chlorite, has caused the growth inhibition seen in these smoky quartz crystals. The chlorite is visible as an icing on the crystals, but the interior is perfectly clear. Chlorite has a tendency to attach primarily to the rhombohedral (r and z) and trapezohedral (x) faces of quartz crystals. Accordingly, the prism faces (m) are largely uneffected and just look a bit dull, while the rhombohedral faces show a very strange surface. They are covered with many small parallely grown crystal tips that stand out above the larger rhombohedral faces. Interestingly, they all grew up to the same level, as can be seen in the upper left edge of the left crystal and the right edge of the right crystal in the second close-up photo. Since the crystals were found completely embedded in chlorite, it also possible that some mineral present in irregular patches within the pocket clay has caused a transitional inhibition on parts of the rhombohedral faces. It is very difficult to say anything with certainty post hoc, however.

This is a possible precursor of an unnamed growth form shown below. From the upper Val Giuv, Tujetsch, Graubünden, Switzerland.

20mm  1808x1552 381kb - 3616x3104 1107kb

These slightly smoky rock crystals carry many parallel grooves, mostly on their rhombohedral faces and only a few on the prism faces. This suggests that the surface pattern is caused by a strange mode of growth. But the grooves are in fact scars caused by a mineral that got dissolved. One can tell because the direction of the grooves does not correlate with a crystallographical axis or the orientation of the crystals, and the grooves line up with grooves of neighboring crystals that have a different orientation, as can be seen on the two neighboring crystals in the lower right corner. Which mineral once filled the grooves is hard to say; platy calcite is very common in the Alpine-type fissures these crystals came from. But some of the grooves are slightly bent, so a fibrous mineral like actinolite or rutile is a better candidate. Amphiboles and rutile do not dissolve easily once formed, but anhydrite does and also forms elongated crystals, and it is more soluble at low than at high temperatures, so these grooves have probably been formed by anhydrite needles that grew in the fissure in the latest phase of quartz crystal growth. This would also explain that the grooves are rather shallow and mainly visible on fast growing rhombohedral and not the prism faces. From the Grimsel area, Berner Oberland, Switzerland.

10mm  1600x2384 222kb - 3200x4768 771kb 10mm  1584x2384 249kb - 3168x4768 853kb 10mm  1536x2000 274kb - 3072x4000 897kb

The following three photos show different views of a very odd crystal from Itremo, Magagascar. It's a typical Itremo milky quartz with split growth on the prism faces, but one half of the crystal tip seems to be cut away. Interestingly, the cut is exactly parallel to one of the a-axes. No clues are left on the crystal as to what caused this, but the zones that are visible on the larger half (see last photo) perfectly match the outline of the smaller half. It is likely that the tip was covered by an growth-inhibiting material on the now smaller half of the crystal - this would work if the crystal was oriented horizontally during growth, with the growth-inhibited parts on the top. Later the material got dissolved and the growth resumed. There are no traces of inclusions visible inside the crystal as phantoms, and there are no indentations left on the surface of the crystal, so a period growth inhibition of the crystal is plausible, but unproven.
Spirifer Collection, Poland.

10mm  1504x2224 494kb - 3008x4448 1508kb

 

10mm  1536x2224 553kb - 3072x4448 1690kb

 

20mm  1790x1472 307kb - 3580x2944 868kb

  Field of view 9cm.

20mm  1200x1632 333kb - 2400x3264 923kb

Growth-inhibited quartz crystals that look like stacks of coins, and pale green datolite crystals. The shape of the quartz crystals is caused by thin calcite plates that started to grow a bit later than the quartz crystals and later got dissolved. Collection Witold Góral.

Needle Quartz

Quartz Perimorph

Quartz perimorphs after carbonates and sulfates, like calcite and baryte, are common in hydrothermal veins, but often it is difficult to determine the older mineral with certainty. Once quartz has formed, it is usually more difficult to dissolve than most other minerals, so a mineral's perimorph after quartz are much rarer.

20mm  1278x1000 166kb - 2556x2000 561kb

Although  this looks a bit like chalcedony covered by small quartz crystals, it is a perimorphosis of quartz after calcite. Platy calcite crystals in a gangue have first been surrounded by quartz. The surface has been encrusted by small quartz crystals. The calcite has later been dissolved. The small yellow rhombohedral crystals are calcite. The specimen is from Herbornseelbach, Hessen, in Germany.

10mm  800x1004 116kb - 1600x2008 336kb

A  rock crystal sits on platy quartz perimorph after an unknown mineral. What is left are hollow plates enclosed by small sparkling quartz crystals. Judging from their shape of the plates this could be a perimorph after platy calcite. From the Gobobseb Mountains, Namibia.

10mm  1224x1024 159kb - 2448x2048 423kb

Quartz  crystals have encrusted pointy scalenohedral calcite crystals that have partially been dissolved. The specimen comes from a hydrothermal calcite vein in the Bleiwäsche quarry at Wünnenberg north of Brilon, Sauerland, Germany.

50mm  1000x784 101kb - 2000x1568 329kb

Despite its rippled and rounded surface structure that is typical for chalcedony, the straight edges and right angles reveal that this specimen is a chalcedony perimorph after a mineral that crystallized as large cubes, most likely fluorite that got dissolved completely. It is interesting that the perimorph is not made of a single, but several nested, more or less complete casts. From Sidi Rahhal, El Kelaâ des Sraghna Province, Marrakech-Tensift-El Haouz region in Morocco.

approx. 50mm  1408x1408 382kb

Another, much larger chalcedony perimorph from Sidi Rahhal, seen at the Hamburg Mineral Fair 2008. Here the cubic shape of the dissolved mineral is obvious. A similar specimen from Morocco that is encrusted with sparkling amethyst is shown on the Tuscon Mineral Fair 2009 poster.

Phantom Quartz

• Minerals have precipitated on the crystal for a short time during crystal growth and have later been embedded in the growing crystal. If minerals precipitate several times during growth, one can see several stacked phantoms, much like Russian Matroska puppets. The phantom is often very faint and ghost-like. This is the classical definition of a phantom.

• Growth conditions that led to the inclusion of colorizing trace elements or minerals have changed. It is not uncommon to find clear quartz with a milky quartz phantom. Other examples would be a smoky quartz phantom inside a colorless rock crystal, or thin zones of amethyst coloration that are parallel to the rhombohedral faces.

20mm  1576x2288 222kb - 3152x4576 614kb

The first specimen represents almost an epitome of a "phantom quartz". It is a slighly smoky crystal with a very nice phantom made of green vermicular chlorite. It is a remarkable piece because the phantom is "complete" and because the phantom reveals that the crystal's shape changed during growth. It is also presented in the Crystal Habits section. From Cerra Do Cabral, Minas Gerais, Brazil.

10mm  1024x1024 104kb - 2048x2048 290kb

A  rock crystal with multiple stacked phantoms caused by fine material that precipitated onto the growing crystal. In the upper left part one can see many fine lines that run parallel to the rhombohedral faces. Left to the center a thin layer of grainy pocket detritus covers a former rhombohedral face. The specimen is from Piz Beverin, Graubünden, Switzerland.

10mm  944x1007 113kb - 1888x2014 388kb

A  small stubby quartz crystal with Tessin habit that includes multiple brown phantoms. These clear phantoms are smoky quartz zones that reflect changes in the chemistry of the environment. The black needles that radiate from the base of the crystal and cover parts of the crystal faces are schörl, a black, iron-rich tourmaline. From a granite miarole pocket in the western Erongo mountains, Namibia.

10mm  1136x1064 233kb - 2272x2128 659kb

This  is a 3 mm thick slice of a quartz crystal prism (cut perpendicular to the c-axis, so the six sides correspond to the prism faces). There is a central citrine core surrounded by alternating zones of rock crystal and smoky quartz. Most of the inclusions (both minerals and fluids) can be found along cracks in the crystal and in irregular patches, but some of them are also concentrated on lines parallel to the prism faces, for example at the upper right and the right edge. The dark vertical lines are large cracks in which various minerals precipitated. The lower colorless part of the crystal that lacks phantoms is actually a part of a neighboring crystal. From the Landsverk I mine at Evje, Aust-Adger, Norway.

5mm  1056x1504 126kb - 2112x3008 386kb

Amethyst phantoms are not uncommon, sometimes together with smoky phantoms, in elongated crystals from Namibia, Madagascar and Mexico. This crystal contains deeply violet but blurred phantoms in the lower part and sharp amethyst-smoky phantoms at the tip. The phantoms are all "complete" and are shaped just like a complete crystal tip, but because they are so thin they can only be seen when viewed along their edges. The crystal is from Ankazobe in Madagascar.

5mm  880x960 110kb - 1760x1920 369kb

This  image shows a ferruginous quartz containing an orange-red phantom made of hematite that precipitated on the crystal faces. The dark metallic spots at the left of the crystal are tiny hematite crystals, too. The specimen is from the Orange River in southern Namibia.

20mm  1120x976 92kb - 2240x1952 256kb

A  typical smoky quartz specimen from Usingen in the Taunus mountains, north-east of Frankfurt, Germany. The crystals lack prism faces (m-faces) and only show rhombohedral faces.

All crystals show a zonar smoky color, forming multiple phantoms. While the look of phantoms caused by the inclusions of minerals can vary among the crystals from a single pocket, phantoms caused by the zonar distribution of color centers (as found in smoky quartz, amethyst, and citrine) show the same pattern in all crystals, as color center formation depends on environmental conditions like the presence and absence of certain elements in the fluids.

10mm  1008x1448 259kb - 2016x2896 715kb

Multiple  phantoms made of stacks of fine layers of chlorite can be seen inside these rock crystals from Minas Gerais, Brazil. The phantoms all lie parallel to the backward rhombohedral faces. The phantoms stand out from this white background because the back sides of the clear and shiny crystals are covered with fine white pocket detritus. As one would expect, all crystals show a Dauphiné habit as this habit is promoted by material that precipitates on just one side of a crystals. In all well-formed crystal tips on this specimen the same pattern of phantoms can be found. Starting from the center of the crystal, there is a dense and dark phantom, followed by a large gap, two nearby groups of chlorite layers, a smaller gap, and finally several fine chlorite layers - all crystals were experiencing the same changes in environmental conditions, of course.

10mm  1568x2288 237kb - 3136x4576 725kb

Often phantoms are hardly visible or they are poorly defined with diffuse borders made of fluid and gas inclusions, like in this interesting rock crystal. The clear tip grew on an older, more stubby crystal. Its phantom is just above the middle of the crystal. The growth did not continue evenly, instead it started at the tip and the crystal grew longer and the second-generation layers were "creeping down" the prism from the tip of the crystal. So one could argue whether this crystal could be called a scepter or not. From the Madem Lakkos Mines, Chalkidiki peninsula, Northern Greece. Collection Anastasios Tsinidis.

Quartz Pseudomorph

So with respect to quartz, there are two possible kinds of pseudomorphoses:

• quartz pseudomorph after some mineral

• some mineral pseudomorph after quartz

The first is actually quartz, the latter is another mineral. The same wording is sometimes used to describe pseudomorphs after non-minerals, for example, a coral agate is said to be a "chalcedony pseudomorph after coral".

Because quartz is more stable in a wide range of geological environments than most other minerals, pseudomorphs of other minerals after quartz are much rarer than pseudomorphs of quartz after another mineral. The latter is often made of chalcedony, not of massive quartz.

According to the most common theory on its genesis, tiger's eye is a "quartz pseudomorph after crocidolite asbestos".

20mm  1168x717 77kb - 2336x1434 223kb

This image shows a pseudomorph of quartz after selenite to the left and a selenite specimen (CaSO₄•2H₂O) in the well-known "gypsum rose" form to the right. The shape of the gypsum rose is well preserved in the pseudomorph, but luster and translucency are different. The quartz did grow as chalcedony, which is a much more common material in pseudomorphs of quartz after another mineral than grainy or massive quartz. The pseudomorph is from Crawford, Dawes County, Nebraska, U.S.A., and the gypsum rose from Touggout in Algeria.

20mm  1132x768 103kb - 2264x1536 325kb

Pseudomorphs of chalcedony after anhydrite (CaSO₄) are frequently found in fissures and geodes in volcanic rocks around Irai, Rio Grande do Sul, Brazil. The typical rectangular cleavage pattern of anhydrite can still be divined on the large fan-shaped blade in the right part of the specimen. To the left are some small dull quartz crystals that grew on top of the former anhydrite blades and are now covered by a thin layer of green chlorite and chalcedony.
Pseudomorphs and perimorphs after anhydrite are quite common because the mineral is more soluble in cold than in hot water, so while the solution in a pocket cools anhydrite gets dissolved, other minerals assume its place. More often anhydrite will take up water and turn into selenite, however.

5mm  640x900 54kb - 1280x1800 163kb

A very unusual pseudomorph of andradite garnet after quartz. One can still see that small crystals were once sprouting from the main crystal, but the entire specimen is made of fine-grained and dull garnet substance. From Stanley Butte, Graham County, Arizona, U.S.A.
Donated by Fred Elsnau.

5mm  1376x2160 330kb - 2752x4320 936kb

Pseudomorphoses of steatite after quartz were found in the Johanneszeche mine at Göpfersgrün, Wunsiedel, but also at a quarry near Thiersheim, both in the Fichtelgebirge mountains in Bavaria. The "crystals" are meticulously preserved and embedded in steatite masses, but - being made of steatite - they are very soft. A description of the localities (in German) is found at Berthold Weber's Mineralienseite: Thiersheim, Göpfersgrün. The specimen on the photo is from Göpfersgrün, donated by Jochen Mattis.

Rainbow or Iris Quartz

One is fairly common: Quartz crystals often show cracks running through them, usually caused by mechanical stress, and these sometimes show rainbow-colored reflections. Due to the conchoidal fracture of quartz the cracks are curved and the width of the fissure is not the same over the whole extent, but changes gradually. The cracks are usually very thin, and often just filled with air. Light that passes through the crack will get partially reflected on both walls of the crack. If the width of the crack is of the same order of magnitude as the wavelength of light, both reflected light beams will show interference colors: Parts of the spectrum will be canceled out, and the reflected light appears colorful. The color depends on the width of the crack and the angle at which the light passes the crack. As the width of the crack changes gradually, so does the color and this causes a rainbow-colored^[2] reflection. This effect is known as iridescence (hence the name iris quartz) and can also be observed when thin layers of oil swim on water.

10mm  1760x1936 394kb - 3520x3872 1768kb

A nice example of an iris quartz with a large colorful and shiny blotch inside. From Dasoto, Kato Nevrokopi Basin, Drama prefecture, Greece. Collection Anastasios Tsinidis.

20mm  2256x1584 330kb - 4512x3168 967kb

The other type of iridescence is much rarer and has first been described by C.V. Raman in 1950. Here the cause of the color is not a crack, but interference by reflection and refraction on parallel twin planes, somewhat related to the effect seen in rainbow or iris agate. While in iris agate a grating made of parallel layers of different refractive indices are the cause, in rainbow quartz it is probably polysynthetical Brazil law twin layers, stacked parallel to the crystal's rhombohedral faces, or by thin cracks that are associated with the twin boundaries. This type of twinning is very common in amethysts, where the individual twin layers are usually large enough to be seen in a thin section under polarized light. They may get very thin, though, and have been the proposed reason for the observed color play in certain crystals from Uruguay, Morroco and India. The Indian specimens are ususally colorless but resemble the typical amethysts from volcanic gas cavities in any other respect, and some of the specimen I have seen do have a faint amethyst color. The image shows such a specimen from Madhya Pradesh, India. The effect is very subtle in most specimen and while it is easily seen on a specimen that is turned in your hand, it is very difficult to capture on a photo. The colored reflections are not at the surface, but come from within the crystal and mix with the regular white reflection from the surface, which gives them a shine that is not unlike but much weaker than that of the infamous aqua aura gold-covered bluish quartz crystals: zones immediately under the rhombohedral faces reflect light in a colorful way, most often in green and pink. Sometimes colored reflections are only seen on parts of a crystal face. In general only a few faces of a specimen show the effect strongly, many faces do not show the effect at all.

The color normally does not change with the angle of the incident light: a rhombohedral face that looks green always looks green, no matter how one turns the crystal, a pink face looks always pink. I've seen photos^[3] of a specimen with faces that show different colors depending on the angle of the incident light at the same face - and this is actually the expected behavior in interference phenomena, where light of different wavelengths are reflected or canceled out depending on the angle of the incident light. A possible explanation: the intensity of reflections at the surface depends on the angle, once the angle of incidence falls under a certain value, the reflection is total, and no light will enter the crystal. Since the twinning planes are parallel to the crystal surface, the reflections get the weaker the more shallow the angle of the incident light is. On the other hand, if the light enters the crystal roughly perpendicular to the surface, the reflections from the surface and the underlying planes are both very weak, and most light passes through. So there's only a narrow range of angles that will cause strong reflections at the twin boundaries and the color of that reflection does not change very much. This, of course, is just an untested proposal to bring the observations in line with the theory. According to Raman (1950), the light transmitted through the reflective patches is almost monochromatic, which is a strong indication of an arrangement of several twinning planes with equal distances. However, the common pink reflections cannot be caused by monochromatic light (pink is not part of the natural rainbow spectrum), only by monochromatic light passing through (and being subtracted from the reflected spectrum). Cut stones seem to show color changes of the reflections more frequently^[4], which would be expected, because the stone's surface is usually not parallel to the reflective zone inside.

20mm  1648x2224 515kb - 3296x4448 1541kb

An amethyst inside a geode showing iridescence, with reflections in various colors from within the crystals. When the specimen is rotated, different crystals light up in other colors. From Sidi Rahhal, Morocco. Spirifer Collection, Poland.

Scepter Quartz

Fig.7: Scepter Quartzes

In all cases, the scepter develops from the already present crystal lattice of the crystal underneath. Thus, to be a scepter quartz, the "second generation" crystal's a- and c-axes need to be oriented parallel to the respective axes of the "first generation" crystal; just one crystal on top of another doesn't make it a scepter. Such a crystallographically well defined intergrowth of different minerals is called an epitaxy. In a sense, a scepter represents an epitaxy of quartz on quartz, and because it is the same mineral, it is sometimes called an autotaxy.

Scepters are quite common in certain geological environments. Amethyst from alpine-type fissures in igneous and highly metamorphosed rocks usually occurs as scepters on top of colorless or smoky crystals (not only in the Alps, but for example also in southern Norway or northern Greece). Here, the amethyst generation grew at lower temperatures than the first generation quartz. The same growth form can be observed in pegmatites and miaroles in igneous rocks (for example, amethyst scepters from the Brandberg, Namibia, or from pegmatites in Minas Gerais, Brazil).

Scepters, or to be precise, the "second generation" part of a scepter quartz that defines it, commonly have a number of morphological properties:

• Scepters are commonly of normal habit and are never tapered. The underlying "first generation" crystal may show a Tessin habit, but the scepter on it will not.

• Scepters tend to assume a short prismatic habit. An apparent exception are reverse scepters and the normal scepters associated with them, which may occur as elongated extensions of a "first generation" crystal, but then in the shape of multiple stacked scepters.

• Many scepters show only a weak striation on their prism faces, sometimes it is even missing.

• Scepters do not show split growth patterns.

• Scepters rarely show trigonal habits with very small or missing z-faces. An exception are reverse scepters and the normal scepters associated with them.

• Scepters are often associated with skeleton growth forms (skeleton or window quartz).

• Scepters commonly show a color, color distribution, diapheny and surface pattern that is markedly different from the underlying "first generation" crystal. Often they are more colorful and transparent. Amethyst scepters are very common, smoky quartz scepters -often with uneven color distribution- are common. An exception are reverse scepters and the normal scepters associated with them which seem to either not differ from the "first generation" or show gradual transitions.

• Summarizing the exceptions above: Reverse scepters and the normal scepters associated with them seem to have a different set of properties.

Formation

One of the problems with that theory is that you would expect to see a larger number of "double", "triple" or "quadruple scepters", specimen in which the growth had been interrupted several times and in which scepters with slowly changing habits are stacked. In nature, however, you see a strong dominance of "simple" scepters that consist of just a prism with "a single head". If you see multiple scepters, then often alongside simple scepters, although multiple changes in the environment should have affected the morphology of all of them equally.

Another problem is that you would not expect to see a fully-grown scepter that encloses the former tip like an onion if the crystal simply started growing from a single point on the surface of the tip. Such a crystal would finally grow into an elongated crystal and would at best assume the shape of a reverse scepter.

As I've mentioned, amethyst from igneous and metamorphic rock locations all over the world predominantly occurs as scepters. Even if you just take Alpine locations, it is hard to imagine that the environmental conditions in all those locations have undergone a single sudden change that led to a temporary growth inhibition on the crystals, followed by a very distinctive growth pattern, the formation of scepters.

The internal structure of scepters from Alpine-type fissures (and of scepters in general) is perhaps always lamellar, as opposed to the macromosaic structure of many quartz crystals from Alpine-type fissures. Quartz crystals with a macromosaic structure may carry a scepter, but the scepter will then show lamellar structure.

5mm   900x916 69kb - 1800x1832 203kb

A  clear, almost colorless scepter that sits between rock crystals is shown in the first picture. This is a Schaumburger Diamant, more about them can be found in the "Diamonds" section. The tiny double terminated crystal grown on the scepter is not a scepter itself, as it is not grown parallel to the crystal underneath.

10mm   800x990 57kb - 1600x1980 179kb

A  very pale amethyst scepter sits on a citrine-colored crystal whose tip is still visible as a yellow phantom. On the bottom there is another smoky phantom, although a bit blurred. This crystal is from the ridge of Peterson Mountain on the Nevada-California border, and it is also shown in the Amethyst section.

10mm   800x1000 75kb - 1600x2000 236kb

This  mushroom-like scepter quartz from a pegmatite deposit appears to have feldspar inclusions in its rhombohedral crystal faces. It is from the Serra Negra, Patroc�nio, Minas Gerais, Brazil.

10mm  720x976 72kb - 1440x1952 220kb

Multiple  scepters can be stacked, of course, as in this specimen from the small scale miners claims south of Tafelkop, Goboboseb-Berge, Namibia. The topmost crystal is not placed centrally on top of the lower crystals, but its c-axis runs parallel to theirs, so it qualifies as a scepter. The specimen is also a skeleton quartz, and the combination of skeleton and scepter growth forms is very common.

5mm  720x984 82kb - 1440x1968 287kb

A  funnel-shaped scepter quartz sitting on a thin stem. It contains liquid inclusions in small irregular bubbles, as well as multiple inclusions of brown clay in the top left area that run roughly parallel to the rhombohedral face. The shape of these clay inclusions indicates that the crystal has for some time been growing as a skeleton quartz. The specimen is from the small scale miners claims south of Tafelkop mountain, Goboboseb-Berge, Namibia.

2mm  800x800 53kb - 1600x1600 157kb

This  small scepter sits in a druzy cavity inside a quartz vein outlined by brownish quartz crystals. It serves as an example form a more cap-like form. The specimen was found on an old pyrite mine dump in the Colline Metallifere hills at Campiano east of Massa Marittima, Tuscany, Italy.

5mm  1000x1392 92kb - 2000x2784 265kb

This  is an example of a scepter that sits sideways on an older smoky quartz prism. The scepter shows a very pronounced zonar coloration with a milky core and amethyst color under the rhombohedral faces and a few red flakes of hematite.
The specimen came from an alpine-type cleft in gneiss near the village of Dasoto in the Kato Nevrokopi Basin, Drama, Northern Greece, at the southern slopes of the Rhodopes. Although it is not from the Alps, it may serve as an example of the very common combination of amethyst color and scepter growth form as it is usually found in Alpine-type fissures. Donated by Anastasios Tsinidis.

10mm  1408x2040 534kb - 2816x4080 1455kb

A small double terminated amethyst clinging to the scarred prism face of a large old smoky quartz of Tessin habit. It is from the same location as the previous crystal, near Dasoto village in Greece, and you can see the similarity in the zonar coloration. Why is this crystal shown in the scepter section? The crystal has a typical "scepter-like habit", it just happened to grow on the prism and not on the tip, but it did so parallel to the crystallographic axes of the older crystal, like all scepters.

10mm  1000x1264 163kb - 2000x2528 460kb

While  an amethyst scepter on smoky quartz or rock crystal is the typical case, one can also find smoky quartz scepters on amethyst stems. This specimen probably came from from a pegmatite or miarole pocket, as there is some feldspar and epidote on the back side. The smoky scepter is not evenly colored but has color zones. The amethyst contains numerous inclusions of shiny red hematite flakes. The smoky scepter is From the Brandberg area, Namibia.

5mm  1024x1528 158kb - 2048x3056 492kb

An  interesting combination of different habits can be seen in this small specimen. The first-generation crystals developed into a Tessin habit with the prism faces being interspersed by steep rhombohedral faces, while the scepter shows a normal habit - as all scepters do. From an alpine-type cleft in mica schist at the Turbenalp, Binntal, Wallis, Switzerland.

10mm  1936x1632 325kb - 3872x3264 921kb

This is a group of reverse amethyst scepters on a volcanic matrix from Bombori, near Potosi in Bolivia. The amethyst is accompanied by white calcite crystals^[5]. The growth form and the paragenesis resemble amethyst specimen from Osilo, Sardinia, and probably nobody would object if I would claim that they actually were from Osilo. There are subtle differences, the shape and color of the calcite crystals differs, the crystals are not evenly colored, and the crystals show inclusions of greenish matrix minerals.

All amethyst crystals on the specimen carry reverse scepter tips, and some of them have pagode-like transitions with several steps. The steps are not perfectly straight, but curved. The number of steps differs between the crystals, so obviously there was not a singular event or change in the environmental conditions that caused the narrowing.

Skeleton Quartz

In German literature this growth form of quartz is known as "Fensterquarz", and the translation, window quartz is gaining popularity, as it is - at least for quartz - a more descriptive term. Occasionally the terms frame quartz and cavernous quartz are used. The term skeleton quartz is indeed a bit confusing as the crystals have not been "skeletized" by dissolution.

The faces on a skeleton quartz will grow from the edges to the center. Sometimes these faces simply grow as thin transparent plates, then usually several generations of them can be found, separated by empty spaces. Should these plates finally be completed, the watery solution inside will be trapped behind a "window".

Skeleton quartz often shows unusual surface patterns and curved distortions of the faces. These might look like fractures but are growth forms.

Skeletal growth is indicative of an unrest geological environment with rapidly falling temperatures and/or pressures. Many skeleton quartzes show inclusions of clay and other detrital pocket material, often captured in large cavities. The sedimentation of fine pocket material takes place quickly compared to the speed of growth of crystals, even that of skeleton quartz, so one would not expect any detrital inclusions in the outer layers of a crystal. The inclusions of clay might be explained by tectonic activity shaking up the fine detritus, or by a boiling watery solution in which this detritus cannot sediment. The fluid and gas inclusions that are often found in skeleton quartz seem to support the latter view, but many biphasic inclusions (that consist of a gaseous and a liquid phase) were monophasic (either liquid or gaseous only) when they were enclosed and became biphasic inside the crystal at lower temperatures.

5mm  800x1000 90kb - 1600x2000 285kb

The  volcanic Goboboseb-Berge (Goboboseb Mountains) in Namibia are renown for nice scepter and skeleton quartz, often amethystine or smoky in color, with inclusions of minerals, fluids, or clay, as the specimen from the small miners claims south of the Tafelkop mountain shown in the picture.

10mm  960x1008 108kb - 1920x2016 336kb

This  smoky scepter quartz from the same location as the first specimen has very nice rectangular windows on its prism faces and triangular windows on the rhombohedral faces.

10mm   834x1004 112kb - 1668x2008 344kb

Another  smoky skeleton quartz from the Goboboseb-Berge in Namibia, showing much more irregular "windows" and large, golden inclusions of clay.

10mm  720x960 100kb - 1440x1920 307kb

Another  skeleton quartz with amethyst color from the area south of the Tafelkop, Goboboseb-Berge, Namibia.

10mm   967x1145 129kb - 1934x2290 333kb

A  double-terminated skeleton quartz from India, showing the typical "windows" on all faces.

20mm  1000x1518 159kb - 2000x3036 468kb

The  Val d'Illiez in Valais/Wallis, Switzerland, is a classical alpine skeleton quartz location. It lies north of the Mont Blanc massive. The crystals can be found in alpinotype vugs in so called "Flysch" sandstone and limestone. The dissolution of organic matter from neighboring slates led to a quickly increasing methane content of the watery solutions. This decreased the solubility of silica and caused a quick growth from an oversaturated solution.

10mm   849x1004 120kb - 1698x2008 364kb

In  this rock crystal the "windows" on the rhombohedral faces are not very deep, but the typical skeleton growth surface structure has led to a rough crystal surface. This crystal is from an alpine-type fissure in the Bündnerschiefer mica schist at the Holzjihorn, Binntal, Wallis, Switzerland.

10mm  1024x1000 125kb - 2048x2000 355kb

Not all crystals in a pocket need to show the same growth forms and habits, as can be seen in this interesting small cluster of rock crystals of various habits. The large central crystal shows typical skeleton growth-patterns on its crystal faces, right before it there is a crystal with normal habit and a flat prism face. The other crystals are of a stubby, barrel-shaped Tessin habit, also called Binntal habit. The specimen is from the Turbenalp, Binntal, Wallis, Switzerland.

5mm  1120x1496 219kb - 2240x2992 578kb

This  skeleton quartz has a bizarre shape - there is even a small hole going through it. Except for the rhombohedral faces at the upper right corner, only skeleton growth surface patterns can be seen on this crystal. Also from the Turbenalp, Binntal, Wallis, Switzerland.

10mm  1048x1048 120kb - 2096x2096 384kb

An  example of a combination of scepter-like and skeleton growth forms and zonar amethyst color. The crystals have a slightly rough surface and are not perfectly clear, but the color is quite deep for its size. The irregular shape at the left side of the specimen and the kidney-shaped indentation on the large rhombohedral crystal face are not damages but typical skeleton quartz growth forms. From the Mörchnerkar area in the Zillertal Alps, Tyrol, Austria.

5mm  1152x1536 177kb - 2304x3072 491kb

While this amethyst is not a typical skeleton quartz, it is a nice example for a typical growth pattern found on skeleton crystals: their sheet-like growth, an indication of their internal onion-like lamellar structure that is commonly found in scepters and skeleton quartzes. The crystal is from an alpine-type fissure at the Kato Nevrokopi basin in northern Greece. Donated by Anastasios Tsinidis.

20mm  1856x1584 362kb - 3712x3168 1011kb

Of all skeleton quartz crystals those found in the vicinity of Poretta Terme in the Italian Appenines seem to be the most shiny ones. They are filled with a very fine light yellow-gray clay and have smooth and lustrous, in part curved crystal faces. Very often they form aggregates of two or more crystals, like these two parallel grown crystals. From Poretta Terme, Emilia Romagna, Italy.

20mm  1728x1504 378kb - 3456x3008 1051kb

The large crystal to the left looks as if it was made of several growth "layers" nested in an onion-like manner, with the layers being separated by bright pocket clay. The "layers" are not separated from each other, of course. From Granaglione, south of Poretta Terme, Emilia Romagna, Italy. Donated by Luigi Agostini.

20mm  1600x2000 377kb - 3200x4000 1043kb

Skeleton quartz frequently appears as crystal aggregates, and often these consist of parallel grown crystals, like this group with the typical pocket clay inclusions from Poretta Terma, Emilia Romagna, Italy.

Star Quartz or Quartz Stars

20mm  2256x1520 658kb 10mm  2160x1424 890kb

Quartz aggregates that are sold as ocean jasper occasionally contain small cavities outlined by green quartz crystals. These belong to star quartz balls that grew from a small chalcedony sphere in their center. This rather unusual specimen is made of many star quartz balls with 1 to 2 cm diameter. Those on the left side still have the yellow chalcedony spherulites in them, on the right side most of them contain just a small spherical cast. The pink vein filling on the top of the specimen is made of agate.

The second image is a close-up of the quartz stars and a cavity. The crystal tips have a trigonal habit and are made of curved rhombohedral faces. Interestingly the center of the quartz stars is more transparent and has a different hue. The crystal tips show a subtle banding, and first this looks a bit like phantoms. But the banding always runs from the upper left to the lower right, irrespectively of the crystals' orientation. This is an indication that the banding predates the crystals and that they grew in an already layered medium, perhaps a silica gel.

From the Morovato Mine, Ambolozo peninsula, Madagascar.

Other Growth Forms

50mm  1568x1834 361kb - 3136x3668 1069kb

This smoky quartz specimen is from the famous pegmatite locality Strzegom in Lower Silesia, Poland, very likely from the Grabina II quarry. There are several plate-like aggregates of roughly parallel grown quartz crystals with milky layers in the center of the plates that look a bit like a "faden". Of course these are not faden quartzes (and one would not expect to find faden quartz in a pegmatite pocket). A closer examination reveals morphological differences to faden quartz: The prism faces of many crystals are resolved into smaller faces, and the crystals are not perfectly parallel.

A possible explanation: Quartz veins had formed in cracks inside an unknown mineral or rock. The mineral or rock got dissolved but the quartz veins withstood the changing conditions, and quartz crystals grew from the both sides of the vein quartz.

20mm  1840x2144 390kb - 3680x4288 1067kb 10mm  1600x1600 228kb - 3200x3200 637kb 20mm  1840x2320 433kb - 3680x4640 1205kb 10mm  1440x2000 385kb

Sometimes the tips of quartz crystals assume a peculiar shape that reminds one of skeletal growth forms: the edges between the rhombohedral faces and sometimes also the edges between the prism faces stand out like ribs. This growth form is most commonly associated with split growth phenomena on sprouting quartz crystals, but also found on cathedral quartz crystals. This can go to the extreme, with star-shaped habits or crystals that have large ribs standing out from the prism like the buttresses found on gothic cathedrals. In Norway shiny crystals of that shape have been found together with hematite near the Tinnsjø lake, Telemark, and are called "rocket quartz" by the locals. Star-shaped, stubby crystals have been found in Madagascar.

This particular crystal is from Itamarandiba, north of Diamantina, Minas Gerais, Brazil. It has a translucent smoky quartz core and the split growth layer and ribs have grown as an almost opaque white secondary layer on top of it. Note that the forementioned crystals of similar growth form from Norway and Madagascar are translucent and shiny.

The second image is a top view of the same crystal that shows that the growth pattern is developed on each of the large rhombohedral faces at the tip of the crystal. The edges between some of the prism faces also have a very weakly developed rib.

The third image emphasizes the split-growth on the prism faces. None of the smaller rhombohedral faces of the sprouting crystals that grew on the prism faces shows the same pattern as the large rhombohedral faces at the crystal's tip, which is different from what can be seen on many artichoke and sprouting quartzes: these show a fractal-like repetition of the patterns over a wide range of scales, as shown on this specimen from Romania.

The surface of the prism faces is rough, very similar to that of scepter quartzes from the Serra Negra, and apparently causes growth inhibition, as shown in the last image, where the individual crystals sprouting out of the prism faces show a mushroom-like habit because an extra, shiny layer of quartz formed only on the rhombohedral faces. At first it looks a bit like a gravitational precipitation of material, but right below the center you see a crystal sprouting in the downward direction, and it looks like an inverted "mushroom", so it is indeed a selective inhibition of growth on prism faces.

I do not have a name for this growth form, but often such crystals are simply called sprouting quartz because of the dominant split growth surface patterns. I also cannot tell if the the development of the frames and ribs is caused or influenced by the split growth.

10mm  1632x2272 393kb - 3264x4544 1088kb

In the German-speaking part of the Alps, collectors call a number of different growth forms "Skeletquarz", including the one shown on the photo. Although this term is a literal translation of the English term "skeleton quartz", it does not necessarily mean the same thing. Most collectors will call the skeleton quartz growth form introduced above "Fensterquarz", while "Skeletquarz" is more often used for growth forms with irregular or jagged surface patterns and irregular crystal shapes that are often caused by partial dissolution of the crystal.

The growth pattern observed on this particular specimen is probably caused by the re-growth of a formerly partially dissolved or growth-inhibited crystal. The crystal is from the Val Maighels, Tujetsch, Graubünden, Switzerland.

10mm  1840x1520 418kb - 3680x3040 1198kb

A strange surface structure developed on this irregularly shaped crystal. It might be the result of some growth inhibition by pocket clay. From an alpine-type fissure at the Rot Wichel mountain, upper Fellital, Gurtnellen, Kanton Uri, Switzerland.

20mm  2080x1412 549kb - 4160x2824 1480kb 10mm  1440x1792 336kb - 2880x3584 924kb

So far I could not come up with a reasonable explanation for this very odd growth pattern. Suggestions are welcome. From an alpine-type fissure at the Secunda Muotta, Val Giuv, Tujetsch, Graubünden, Switzerland.

Footnotes

2 "Rainbow colored" is actually not correct. There are pink reflections, and pink is not part of the natural rainbow spectrum. The colors appear because parts of the spectrum are canceled out, so some colors are subtracted: If you subtract green from white, you will get the pink that is absent from a rainbow.

3 Thanks to Yuko Tanaka, Colorstone Mithrill, Japan, for pointing this out. I initially thought that the color does not change at all because I did get some parts of the interference mechanism wrong.

4 Thanks to Yuko Tanaka, Colorstone Mithrill, Japan, for for providing photos of cut stones.

5 To the collector of Bolivian minerals this is the most interesting thing about the specimen: calcite is extremely rare in Bolivia.

Source: http://www.quartzpage.de/print/gro_text.html

Home: www.quartzpage.de