Rock Crystal

Rock Crystal is the name given to all clear colorless quartz. It is widely used as a popular ornamental stone and is also used as a gemstone. Although it is one of the least expensive gemstones, cut rock crystal has been used as imitation diamonds. Rock crystal lacks the fire, color (of course) and the rarity to be ranked as a fine precious gemstone. None-the-less, rock crystal is in wide use as a gemstone due to its beauty, affordability, availability, and ease of cutting.

The most common use for rock crystal is in ornamental carvings. A well known ornamental carving for rock crystal is the crystal ball of fortune telling fame. While rock crystal is common it is hard to find large crystals of quartz with the clarity and size required for the crystal balls that most of us associate with fortune tellers. Rock crystal is used for many ornamental carvings from spheres (crystal balls) to pyramids to obelisks to figurines to eggs to bowls to wands. There are also many fine chandeliers that are outfitted with rock crystal ornaments.

Rock crystal can have the colorless clarity of pure water, unlike ordinary window glass which is a pale green. Yet the most common flaws - internal fractures - result in veils and colorful refractions that lend beauty to an otherwise ordinary object.

Rock crystal is an alternate and traditional birthstone for the month of April. Quartz is often associated with balance, clarity, and energy.
Many people prefer uncut clusters of rock crystal. These natural treasures look like crystal cities of fantasy stories. The long slender clear prisms of quartz projecting upward from a common base are true mineralogical wonders that can be admired by all. They cost of good clusters of rock crystal is quite affordable and comes from sources around the world. The best rock crystal sources are in the famous Hot Springs area of Arkansas, USA; Cumberland, England; St. Gotthard, Switzerland; Brazil and Madagascar. Large individual crystals of quartz have been found in Brazil, the largest weighs over 44 tons.

rutilated quartz

Rock crystal often has inclusions of other minerals and these inclusions sometimes produce popular varieties of ornamental stone. Golden rutile inclusions produce a unique stone aptly named rutilated quartz that has a very hair-like look.

tourmalinated quartz

Another apply named stone is called tourmalinated quartz and contains intricately crossing needles of black schorl tourmaline trapped in the clear crystal.

Phantoms are a result of inclusions which occur when other minerals such as chlorite, hematite or even milky quartz form as a crust on the surface of the crystals during a hiatus in the crystal's growth. The crystal then grows out and around the encrusting growth, encasing it inside. But since the thin encrustations formed over a crystal complete with crystal faces it appears that there is a crystal inside the crystal. The encrustations are often incomplete or diffuse and appear ghostly, hence the name phantom.
Rock crystal is only one of several quartz varieties. Other varieties that form macroscopic (large enough to see) crystals are as follows:

• Amethyst is the purple gemstone variety.
• Citrine is a yellow to orange gemstone variety that is rare in nature but is often created by heating Amethyst.
• Milky Quartz is the cloudy white variety.
• Prasiolite is the leek-green variety.
• Rose quartz is a pink to reddish pink variety.
• Smoky quartz is the brown to gray variety.

Quartz is the most common mineral on the face of the Earth. It is found in nearly every geological environment and is at least a component of almost every rock type. It frequently is the primary mineral, >98%. It is also the most varied in terms of varieties, colors and forms. This variety comes about because of the abundance and widespread distribution of quartz. A collector could easily have hundreds of quartz specimens and not have two that are the same due to the many broad categories. The specimens could be separated by answers to the following questions: color?, shade?, pyramidal?, prismatic?, druzy?, twinned?, sceptered?, phantomed?, included?, tapered?, coated?, microcrystalline?,stalactitic?, concretionary?, geoidal?, banded?, etc. Multiple combinations of these could produce hundreds of unique possibilities.

Cryptocrystalline (crystals too small to be seen even by a microscope) varieties are also used as semi-precious stones and for ornamental purposes. These varieties are divided more by character than by color.

The primary varieties of chalcedony are as follows:

• Agate is a banded variety (sometimes with translucent bands)
• Bloodstone is green with red speckles
• Carnelian is yellow to orange
• Chrysoprase is green
• Flint is generally black with a fibrous microscopic structure
• Jasper is any colorful agate
• Onyx is black, white, or alternating black and white
• Sard is yellow to brown
• Sardonyx is banded, alternating sard and (usually white) onyx

Quartz is not the only mineral composed of SiO₂. There are no less than eight other known structures that are composed of SiO₂. These other substances and quartz are polymorphs of silicon dioxide and belong to an informal group called the Quartz Group or Silica Group. All members of this group, except quartz, are uncommon to extemely rare on the surface of the earth and are stable only under high temperatures and high pressures or both. These minerals have their own unique structures although they share the same chemistry, hence the term polymorph, which means many forms.

Quartz has a unique structure. Actually, there is another mineral that shares quartz's structure, and it is not even a silicate. It is a rare phosphate named berlinite, AlPO₄, that is isostructural with quartz. The structure of quartz involves corkscrewing (helix) chains of silicon tetrahedrons. The corkscrew takes four tetrahedrons in order to repeat itself, or three turns. Each tetrahedron is essentially rotated 120 degrees. The chains are aligned along the C axis of the crystal and interconnected to two other chains at each tetrahedron making quartz a true tectosilicate. This structure is not like the structure of the chain silicates or inosilicates whose silicate tetrahedronal chains are not directly connected to each other. The structure of quartz helps explain many of its physical attributes.

For one, the helix makes three turns and this helps produce the trigonal symmetry of quartz. Likewise a helix or corkscrew lacks mirror planes of symmetry as does quartz. The corkscrew structure would also disrupt any cleavage which requires a plane of weakness not found in quartz and breakage would result in the curved fracture, conchoidal, that is found in quartz. Quartz can also have left and right handed crystals just as a corkscrew can screw in a left handed way or in a right handed way. There are even some very difficult to identify crystals of quartz that are twinned with alternating one sixths of the crystal being right handed and then left handed.

Quartz is a fun mineral to collect. Its abundance on the Earth's surface is incredible and produces some wonderful varieties that don't even look like the same mineral. A collector must always be up on the many varieties of quartz and it sometimes embarrasses a collector to have collected too many specimens of such a common mineral. But nearly all collectors concede that you can never really have enough quartz specimens. Note that quartz is nearly a defining component of most geodes. The agate form typically lines the original cavity, creating a durable shell to contain subsequent mineral growth, which itself is often quartz crystals of one or more varieties. Geodes from Brazil are a good example, as layers of agate comprise the shell, lined with milky and/or colorless quartz crystals, topped with amethyst.

PHYSICAL CHARACTERISTICS:

• Color is as variable as the spectrum, but clear quartz is by far the most common color followed by white or cloudy (milky quartz). Purple (Amethyst), pink (Rose Quartz), gray or brown to black (Smoky Quartz) are also common. Cryptocrystalline varieties can be multicolored.
• Luster is glassy to vitreous as crystals, while cryptocrystalline forms are usually waxy to dull but can be vitreous.
• Transparency: Crystals are transparent to translucent, cryptocrystalline forms can be translucent or opaque.
• Crystal System is trigonal; 3 2.
• Crystal Habits are again widely variable but the most common habit is hexagonal prisms terminated with a six sided pyramid (actually two rhombohedrons). Three of the six sides of the pyramid may dominate causing the pyramid to be or look three sided. Left and right handed crystals are possible and identifiable only if minor trigonal pyramidal faces are present. Druse forms (crystal lined rock with just the pyramids showing) are also common. Massive forms can be just about any type but common forms include botryoidal, globular, stalactitic, crusts of agate such as lining the interior of a geode and many many more.
• Cleavage is very weak in three directions (rhombohedral).
• Fracture is conchoidal.
• Hardness is 7, less in cryptocrystalline forms.
• Specific Gravity is 2.65 or less if cryptocrystalline. (average)
• Streak is white.
• Other Characteristics: Striations on prism faces run perpendicular to C axis, piezoelectric (see tourmaline) and index of refraction is 1.55.
• Associated Minerals are numerous and varied but here are some of the more classic associations of quartz (although any list of associated minerals of quartz is only a partial list): amazonite a variety of microcline, tourmalines especially elbaite, wolframite, pyrite, rutile, zeolites, fluorite, calcite, gold, muscovite, topaz, beryl, hematite andspodumene.
• Notable Occurrences of amethyst are Brazil, Uraguay, Mexico, Russia, Thunder Bay area of Canada, and some locallities in the USA. For Smoky Quartz; Brazil, Colorado, Scotland, Swiss Alps among many others. Rose Quartz is also wide spread but large quantities come from brazil as do the only large find of Rose Quartz prisms. Natural citrine is found with many amethyst deposits but in very rare quantities. Fine examples of Rock crystal come from Brazil (again), Arkansas, many localities in Africa, etc. Fine Agates are found in, of course, Brazil, Lake Superior region, Montana, Mexico and Germany.
• Best Field Indicators are first the fact that it is very common (always assume transparent clear crystals may be quartz), crystal habit, hardness, striations, good conchoidal fracture and lack of good cleavage.