Varieties (according to color)
Pure quartz, traditionally called rock crystal (sometimes called clear quartz), is colorless and transparent (clear) or translucent. Common colored varieties include citrine, rose quartz, amethyst, smoky quartz, milky quartz, and others. Quartz goes by an array of different names. The most important distinction between types of quartz is that of macrocrystalline (individual crystals visible to the unaided eye) and the microcrystalline or cryptocrystalline varieties (aggregates of crystals visible only under high magnification). The cryptocrystalline varieties are either translucent or mostly opaque, while the transparent varieties tend to be macrocrystalline. Chalcedony is a cryptocrystalline form of silica consisting of fine intergrowths of both quartz, and its monoclinic polymorph moganite.[ Other opaque gemstone varieties of quartz, or mixed rocks including quartz, often including contrasting bands or patterns of color, are agate, sard, onyx, carnelian, heliotrope, and jasper.
Citrine is a variety of quartz whose color ranges from a pale yellow to brown. It is nearly impossible to tell cut citrine from yellow topaz visibly. Citrine has ferric impurities, and is rarely found naturally. Most commercial citrine is in fact artificially heated amethyst or smoky quartz. Brazil is the leading producer of Citrine, with much of its production coming from the state of Rio Grande do Sul.
Citrine is one of three traditional birthstones for the month of November.
Rose quartz is a type of quartz which exhibits a pale pink to rose red hue. The color is usually considered as due to trace amounts of titanium, iron, or manganese. Some rose quartz contains microscopic rutile needles which produces an asterism in transmitted light. Recent X-ray diffraction studies suggest that the color is due to thin microscopic fibers of possibly dumortierite within the massive quartz.
In crystal form (rarely found) it is called pink quartz and its color is thought to be caused by trace amounts of phosphate or aluminium. The color in crystals is apparently photosensitive and subject to fading. The first crystals were found in a pegmatite found near Rumford, Maine, USA, but most crystals on the market come from Minas Gerais, Brazil.
Rose quartz is not popular as a gem - it is generally too clouded by impurities to be suitable for that purpose. Rose quartz is more often carved into figures such as people or hearts. Hearts are commonly found because rose quartz is pink and an affordable mineral.
Amethyst is a popular form of quartz that ranges from a bright to dark or dull purple color.
Smoky quartz is a gray, translucent version of quartz. It ranges in clarity from almost complete transparency to a brownish-grey crystal that is almost opaque. Some can also be black.
Milk quartz or milky quartz may be the most common variety of crystalline quartz and can be found almost anywhere. The white color may be caused by minute fluid inclusions of gas and/or liquid trapped during the crystal formation. The cloudiness caused by the inclusions effectively bars its use in most optical and quality gemstone applications.
Varieties (according to microstructure)
Although many of the varietal names historically arose from the color of the mineral, current scientific naming schemes refer primarily to the microstructure of the mineral. Color is a secondary identifier for the cryptocrystalline minerals, although it is a primary identifier for the macrocrystalline varieties. This does not always hold true.
|Chalcedony||Cryptocrystalline quartz and moganite mixture. The term is generally only used for white or lightly colored material. Otherwise more specific names are used.|
|Agate||Multi-colored, banded chalcedony, semi-translucent to translucent|
|Onyx||Agate where the bands are straight, parallel and consistent in size.|
|Jasper||Opaque cryptocrystalline quartz, typically red to brown|
|Aventurine||Translucent chalcedony with small inclusions (usually mica) that shimmer.|
|Tiger's Eye||Fibrous gold to red-brown coloured quartz, exhibiting chatoyancy.|
|Rock crystal||Clear, colorless|
|Citrine||Yellow to reddish orange to brown, greenish yellow|
|Prasiolite||Mint green, transparent|
|Rose quartz||Pink, translucent, may display diasterism|
|Rutilated quartz||Contains acicular (needles) inclusions of rutile|
|Milk quartz||White, translucent to opaque, may display diasterism|
|Smoky quartz||Brown to grey, opaque|
|Carnelian||Reddish orange chalcedony, translucent|