Quartz

Chemical composition -- Silicon dioxide.

Quartz is one of the most abundant minerals on Earth and is found in many different forms almost everywhere. There are two major classifications of quartz: crystalline quartz and cryptocrystalline quartz.

Color -- Very wide color range -- colorless, yellow, brown, purple, pink, greenish. Cryptocrystalline quartz often displays exotic color bands, swirls, and other patterns.

Optics -- R.I. 1.553-1.554. Uniaxial positive.

Durability -- Hardness 7.

Crystal structure -- Hexagonal.

Specific Gravity -- 2.651 for crystalline material, up to 2.91 for cryptocrystalline.

Sources -- Extremely widespread.

Quartz (Crystalline)

Crystalline quartz is quartz that occurs in distinct crystals. It occurs in a number of familiar varieties distinguished by color.

Amethyst is purple quartz and is one of the most popular gemstones. It varies from pale lavender ("Rose-de-France" amethyst) to deep purple with red highlights ("Siberian" amethyst). Amethyst is generally abundant and quite inexpensive (pennies to a few dollars per carat), although fine, large Siberian amethyst is rather scarce and may command prices of several tens of dollars per carat. When exposed to strong sunlight for extended periods, amethyst may fade in color. Recently, gem markets have been flooded with inexpensive synthetic amethyst. Although such synthetics can usually be identified by testing for twinning, routine testing is not cost-effective, and some new synthetics do not show the twinning effect.
Citrine is yellow to brown quartz. It varies from pale yellow to rich golden yellow to dark orange. Although citrine may occur naturally, much is produced by heating amethyst under controlled conditions (overheating drives off all color, leaving colorless rock crystal). Darker colors are more highly prized, including medium golden orange ("Rio Grande" citrine) and dark sherry-colored ("Madeira" citrine).
Ametrine is a combination of amethyst and citrine within a single crystal and was discovered only a few years ago in Bolivia. Such bicolored stones are fairly unusual and can be very striking in appearance, but prices remain rather low. For a brief period, these stones were erroneously thought to be manmade or treated to produce the bicolor effect, but it is now known to occur naturally.
Vermarine, also known as Prasiolite or "Greened" Amethyst is a light to medium green quartz produced by careful heating of amethyst from one Brazilian location. Although inexpensive, it is seldom seen and remains something of a collector's item.
Rock Crystal is clear colorless quartz. Its value tends to be very low, except for large flawless pieces, which are rare. One of the finest known pieces is the 12.75 inch diameter, 107 pound flawless sphere ("crystal ball") in the collection of the Smithsonian Institution.
Rose Quartz is pink quartz. It is usually translucent to semitransparent. Strong rose-red, transparent material is rare. Like amethyst, it may fade on prolonged exposure to strong sunlight.
Smoky Quartz is tan or brown quartz. Very dark reddish brown smoky quartz is known as cairngorm (from the Cairngorm Mountains in Scotland), and black or blackish brown material is known as morion.

Cryptocrystalline Quartz

Cryptocrystalline quartz is quartz in which the crystals are microscopic in size and thus is always opaque or translucent. It occurs in a huge array of colors and patterns and, like crystalline quartz, occurs in myriad locations.

Agate is a chalcedony that displays an incredible variety of color patterns -- generally curved bands of regular or irregular formation. It occurs in myriad locations, and some forms are rare and relatively expensive.

Aventurine is a more or less colorless chalcedony that contains uniformly dispersed flakes of greenish mica, thus giving the stone a characteristic speckled green appearance known as aventurescence.

Bloodstone, also known as heliotrope, is a dark green chalcedony or jasper with flecks of red.

Carnelian or cornelian is a reddish brown chalcedony. In ancient Rome, it was often used in cameos and intaglios.

Chalcedony is a translucent or semitranslucent cryptocrystalline quartz, which may be patterned (agate) or uniform in color (blue, green, pink, black, white, etc.).

Chrysocolla quartz is chalcedony mixed with varying amounts of chrysocolla. The chrysocolla provides the robin's egg blue coloration, and the quartz provides sufficient hardness for use in jewelry. The durability varies with the corresponding proportions of the two components.

Chrysoprase is a light to medium, slightly yellowish green chalcedony. Australia provides many of the finest examples.

Fire Agate is chalcedony that contains many layers of tiny inclusions of limonite or goethite, which produce a distinctive firelike iridescence when properly cut to leave only thin protective layers of chalcedony over the inclusions. Many of the finest specimens have been found in Arizona.

Jasper, in contrast to chalcedony, is an opaque and more coarsely grained cryptocrystalline quartz. Like chalcedony, it may be patterned or uniform in color.

Picture jasper, or scenic jasper, may display quite realistic depictions of natural scenes, animals, or other objects, and can command respectable prices due to such unusual pictures.

Onyx is something of a catchall term that usually refers to dyed black chalcedony ("black onyx"), but it is also used to describe other colors of dyed chalcedony. More strictly speaking, onyx agate is banded agate displaying straight and parallel bands of alternating colors, which has historically been used to produce cameos. (See sardonyx.) True onyx, however, is a form of calcite, usually used in architecture and ornamental pieces.

Petrified Dinosaur Bone or "dinny bone" is the product of millenia-old fossilized bone (from dinosaurs!) in which the cellular contents have been replaced by quartz, leaving the cellular bone structure intact. The deserts of the American southwest (notably Utah) produce many fine specimens.

Petrified Wood, like petrified bone, retains the cellular structure of the original wood, but the cellular contents have been replaced by quartz.

Prase is a light yellow-gray-green chalcedony.

Sard is a darkish brown chalcedony similar to carnelian but of less intense color.

Sardonyx is an agate with alternating straight, parallel bands of reddish brown and, usually, white.

Tigereye is a yellowish brown cryptocrystalline quartz in which quartz has replaced crocidolite, a fibrous, asbestos-like mineral. The parallel fibrous structure produces a characteristic chatoyance (silky sheen), or even a cat's-eye effect. Gray, green, and blue (hawk's-eye) tigereye are also used as gemstones; they are usually the result of chemical or heat treatment. Tigereye is often used for cameos and intaglios.

Turritella is an agglomeration of fossilized turritella snail shells replaced by silica. Wyoming is the principal source.

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