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"Rock Crystal" is the name for clear, colorless quartz. In a sense, rock crystal is the ideal image of quartz.
The term crystal goes back to the Greeks, who once thought that since the crystals came from high in the mountains, they might be ice frozen so deeply that it is can't melt again (another interpretation says they meant crystals to be some substance frozen very deeply, but not necessarily water). Of course it was already known that rock crystals can also be found at places that never see snow or ice. But the lack of a better theory let the bad theory survive.
The specimen to the right is from the Kullu Valley, Himachal Pradesh, India.
Note: Rock crystal translates into Bergkristall (mountain crystal) in German.
In the Alps, the notion "Bergkristall" is associated with colorless, clear, long, prismatic crystals attached to the host rock. So a Herkimer diamond would not be called a Bergkristall and short-prismatic colorless quartz from druses wouldn't either. The notion of Bergkristall is not only more specific in physical terms than "rock crystal", the word also has a "romantic" or "mythic" sound in it, not unlike "Edelweiss".
Of course there is no strict rule, and you may call a Herkimer "Bergkristall" if you want.
Rock crystals have also been a source of income in the central Alps since Roman times. To give you an idea of how much rock crystal is part of the alpine culture: If you drive up the Furka Pass from the Upper Wallis in Switzerland, you will see an engraving of rock crystals in the concrete walls in one of the curves.
Specific Properties
Except for being colorless and clear, there is no feature specific to rock crystal that cannot be found in other macrocrystalline varieties.
Rock crystal from many locations can be artificially turned into smoky quartz by high energy radiation. The aluminum necessary for the formation of smoky quartz color centers is already present in sufficient amounts in these rock crystals.
Occurrence
Rock crystal can be found in some form in any geological environment that allows the formation of quartz in general. Large, well-formed and transparent crystals can be found in some pegmatites, in alpinotype vugs, and hydrothermal veins. For nice crystals to form, conditions in a geological environment need to change more or less gradually and slowly over a long period of time. A single rock crystals from the Alps may have grown over a period of several million years during the uplift and folding of the mountains.
Locations and Specimen
Rock crystal can be found at many places all over the world, but few are commercially exploited:
Ouachita Mountains in Arkansas, U.S.A.
Herkimer Area, New York State, U.S.A.
Minas Gerais, Brazil
Polar Ural, Russia
Madagascar
Peru ?
Nepal
Of the Strahlers in the Alps, very few can make a living from selling minerals, and Strahlen always used to be only a secondary source of income for the local farmers. A lot of specimen enter the market as a byproduct of other mining operations, for example from:
China (Iron Ore & Fluorite)
Peru (Lead and Zinc ore)
The chances of getting nice specimen from mines in developed countries were once much better, because the working conditions were different: a lot of work was done manually and nice finds were put aside by the miners, whereas nowadays large parts of the work is done by autonomous machines.
Bulgaria
These
rock crystals from Madan, Bulgaria, grew in a vein along with sphalerite (metallic black) and dolomite (light reddish-brown). Rock crystals can be found frequently in certain ore deposits.
Brazil
Currently the most important exporter of rock crystals is Brazil. Most of them come from pockets and quartz veins in pegmatites in the state Minas Gerais. On Toni Imhof's Web page you can find a report (in German) about
Corinto, Minas Gerais, probably the best known Brazilian rock crystal location.
These
rock crystals are from Gouveia, about 30 km south of Diamantina, Minas Gerais. As you can tell from the name of the district capital, this is also one of Brazil's diamond mining areas.
There is a very faint phantom in the large central crystal.
A
large crystal from Corinto, Minas Gerais, about 180 km north of Belo Horizonte. It has grown on a thin quartz matrix. Quartz from the Corinto area often shows peculiar patterns on the clear prisms, as can be seen on the back side of the large crystal, the small one pointing to the right.
These
quartz crystals from Joaquim Felício, Minas Gerais, about 200 km east of Montes Claros, show an unusual crystal face on three of the six sides of the prism. The thinner, darker prism face in the center of the large crystal is one of the three m-faces, and the two neighboring faces are so called
steep rhombohedra with their typical surface structure. The morphology of these crystals and the underlying crystal forms are explained in the
Crystal Forms section.
China
Lots of rock crystal comes from china, mostly from mines in ore deposits.
Clear
rock crystals on hematite roses, Daya, Ganzhou, Jianxi Province. Many "cuts" run through the crystals, traces of hematite roses that hindered the growth of the quartz and that disappeared later. In the central right part of the image you can see small pieces of hematite that are still present in the crystals.
France
The
specimen to the right was sold as being from Grenoble. The crystals show a so-called Dauphiné habit, with a single prominent rhombohedral face at the tips. The Dauphiné is the old name for the French Alps around Grenoble. Some of the best Alpine rock crystal specimen came from a classical locality 50km south-east of Grenoble: the old gold mine La Gardette, at Villard Notre-Dame, south of Bourg D'Oisans, that has also been exploited for rock crystals since the early 18th century. This is also the location where Japan law twins have first been described as "La Gardette twins".
Germany
In
the village of Ramsbeck, Sauerland, there is a famous lead zinc mine that was still in operation in the 1970s, but now is a mining museum. Many interesting minerals have been recovered from the mine, and of course also rock crystal. The typical Ramsbeck rock crystal specimen on the image shows nice prismatic rock crystals intergrown with saddle shaped dolomite and chalcopyrite. Ramsbeck is also renown for its "Messerquarz" ("knife quartz"), platy faden quartz, presented in the
faden quartz section of the growth forms chapter.
These
small rock crystals from the Piesberg mountain near Osnabrück, Lower Saxony, qualify as needle quartz. They are from a quarry in sedimentary rocks from the carbon ages, a location that is actually more known for its plant fossils. Many of the perfectly clear crystals have developed a Dauphiné habit, which is a bit unusual for quartz from sedimentary deposits. There are signs for an uplift of the Piesberg of about 2000 m, so their formation has perhaps taken place at much greater depth and under the influence of a nearby (but deeply situated) magmatic intrusion, the "Bramscher Intrusiv".
The
term "rock crystal" also includes quartz crystals that have very little in common with the ideal image of an Alpine "Bergkristall". These tiny stubby double terminated quartz crystals grew on top of dog-tooth calcite crystals in a hydrothermal vein in limestone that was mostly filled with calcite. The largest crystal does not even measure 3 mm, but the crystals are eye-catching, because they are much more shiny than the mostly dull calcite crystals. From a limestone quarry at Suttrop, Warstein, Sauerland. Crystals from that and similar locations in sedimentary rocks never show any accessorial faces (like s- or x-faces, for example) and are usually twinned.
India
Note: The following specimen were once presented as being from Nepal, and from Arkansas, U.S.A., and I bought them from two different dealers. The first specimen was labeled "Korur, Indian Nepal", and after talking to the dealer that sold me that piece, it was meant to be from India, not Nepal. The first dealer had more specimen from India with a striking similarity to the one labeled "Mount Ida, Arkansas", with characteristic x-faces that are rather uncommon in Arkansas quartz. However, I could not find a "Korur" on any map of India. After comparing these with other specimen at Indian dealers and talking to collectors who have visited the area, I'm certain that both specimen are from the Kullu Valley, Himachal Pradesh.
A
group of clear rock crystals from the Kullu Valley, Himachal Pradesh. The Himalaya has just recently been discovered as a source of excellent rock crystals. Lots of material comes from alpine-type fissures in gneisses and metamorphosed schists of the Ganesh Himal region north of Kathmandu in Nepal, but rock crystal can apparently be found along the whole west-eastern extension of the Himalayas. This specimen comes from quartz veins in quartzite, and I do not know yet if these are alpine-type fissures or "ordinary" hydrothermal quartz veins.
This
small group of quartz crystals also comes from the Kullu Valley, Himachal Pradesh. Three perfectly clear left-handed crystals stand out and show the typical fine striation on the prism faces. The specimen has a flat bottom with a grainy surface, and was retrieved from a vein in quartzite.
The larger crystals carry very large x-faces that replace a big portion of the frontal prism faces and lack s-faces. The forms on these crystals are discussed in the Crystal Forms section.
Italy
This
double terminated rock crystal is a faden quartz from Mont Chaz Dura near the Piccolo San Bernardo pass at the border to France in the Italian Alps. The "faden" (German for "thread") is the white cloudy structure in the middle of the crystal pointing upwards. It's indicating that the crystal was initially attached to opposite walls of the vug while it was slowly widening. More about faden quartz can be read in the
growth forms section.
Madagascar
These
rock crystals from Antsirabé have a most interesting surface structure, possibly due to etching, followed by incomplete healing. A detailed view of the left crystal's tip is shown in the "growth forms" section, where I will also discuss possible causes of the brick-like pattern on the rhombohedral faces.
Rock
crystals from Amputofiamendia in Madagascar with unusually large bipyramidal diamond-shaped s-faces. Similar to the specimen from Antsirabé, there is still a lot of a reddish, very sticky powder attached to it, something that you can see at many rock crystals from Madagascar.
Peru
A
typical specimen from a Peruan mine, elongated rock crystals on nice pyrite cubes and black sphalerite.
Switzerland
A
very beautiful group of Dauphiné habit rock crystals. They are perfectly clear, but as it is typical for Dauphiné habit crystals, their back side is covered with smaller shiny rock crystals that precipitated on the prism face. Some of those that developed into larger crystals peek out behind the prism of the large crystal. The specimen comes from an alpine-type fissure in gneiss at the Halsensee, Binntal valley, Wallis.
A group
of Dauphiné habit rock crystals from the same alpine-type fissure at the Halsensee as the previous specimen. The crystals are clear and colorless but covered by a thin yellow coating of iron oxides and hydroxides that has intentionally been left on the specimen. Such a yellow is very common on rock crystals from many different environments, but is usually removed in an acid bath.
This
is so called needle quartz from the Val Bedretto, Ticino. Most of the crystals on this specimen have developed a Dauphiné habit with one of the rhombohedral faces enlarged. This specimen is also described as
needle quartz in the "Growth Forms" section.
Small
rock crystals on dolomite matrix from Wyssi Flue in the Binntal, Wallis. The location is just 2 kilometers east of the famous Lengenbach pit. The quartz crystals occur in long fissures that cut through the dolomite rock.
A
rock crystal with Tessin habit from the Fäldbachtal, Binntal, Wallis. While an "ideal" Tessin crystal has very small rhombohedral faces or lacks the altogether, such crystals from the Binntal often show well developed rhombohedral faces.
This
rock crystal from the Turbe-Alp, Binntal, Wallis, shows the typical Binntal habit that can be seen at many specimen from this valley. It is a Tessin habit that has a very wide prism. The typical Tessin habit crystal has an elongated look, while crystals of Binntal habit look rather short and have relatively large rhombohedral faces.
A
group of small parallel-grown crystals with Binntal habit from the Holzjihorn in the upper Fäldbachtal, Binntal, Wallis.
A
small group of rock crystals from southern slope of the Holzjihorn, Feldbächtal, Binntal, Wallis. The specimen was retrieved from a pocket in "Bündnerschiefer", a weakly metamorphosed mica schist that has been subject to enormous stress and strain during the uplift and folding of the Alps, leading to the formation of alpinotype vugs. The rough surface of the mostly clear crystals is caused by skeleton growth patterns.
A
rock crystal that is both in between a Binntal and a normal habit and a skeleton quartz. The "windows" on the rhombohedral faces that are so typical for skeleton growth are not very deep, but the skeleton growth has led to a roughening of the crystal surface. This crystal is from the same pocket as the specimen in the preceding image.
U.S.A.
When talking about rock crystal locations in the U.S.A., two places immediately come to mind which are both presented in their own chapters:
Herkimer in New York State, and the
Ouachita Mountains in Arkansas.
These
crystals grew in a small geode in basalt in the Mopah Range inside the Turtle Mountain Wilderness, Southern California, and rock crystals like these are not what you would expect in a volcanic rock. The geode wall is made of calcite. Most of the crystals are scepter quartzes, but the scepters are more elongated than usual, so it is a bit difficult to recognize them.
Further Information, Literature, Links
A lot of information can be found about rock crystal locations in Arkansas. A good source is the quartz page on
Rockhounding Arkansas.
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