There's a small pun in this category name; obviously, quartz is a rock but the category name refers to my enthusiasm with the beautiful variety by this most awesome gemstone. The base chemical structure is always the same (SiO2): silica tetrahedra with shared corners, increasing the silicon to oxygen ratio to 1:2 instead of 1:3 or 1:4. There's the "macrocrystalline" quartz in which individual crystals are visible to the naked eye (think clusters of prismatic crystals in a geode); examples include amethyst, citrine, rose quartz and smoky quartz. "Microcrystalline" or "cryptocrystalline" quartz show individual crystals under magnification and tend to be more translucent to opaque; examples include agate, carnelian, chalcedony and jasper. Quartz is the second most abundant mineral in the Earth's crust, after feldspar, so it is found everywhere. Trace amounts of elements like titanium (rose quartz), iron oxide (citrine and amethyst), hematite (red jasper) and nickel (chrysoprase) give the stones their color. For colorful stones like banded agate, jasper and sardonyx, the colors form as a result of slight element changes during the deposition of the silica.

•• a g a t e

Agate is a type of chalcedony that is characterized by various patterns. Most agate is banded in concentric forms, but it can also form with dendrite-like inclusions (moss agate). Banded agate may be simple, as in Brazilian agate with concentric circles, or convoluted like the "crazy lace" agate found in Mexico. Blue-lace agate from South Africa is pale blue with delicate layers of lighter and darker hues. Fire agate includes red to brown hematite which goes the stones inner iridescence. A common place to find moss agate is Medford, Oregon. The Petrified Forest in Arizona is agatized wood in which the organic matter has been replaced by agate. Agate is often dyed in vibrant colors that highlight patterns in the stone.

•• a m e t h y s t

Amethyst has long been associated with royalty and purity; deep purple stones tend to be the most valuable, but lavender amethyst can also be lovely. The name comes from Greek for "not drunk", referring to the myth in which Dionysus created amethyst in repentance for sending tigers to devour a young maid. It was thought that drinking from amethyst cups would protect the drinker from drunkenness. On the opposite end of the spectrum, the early Christian church adopted amethyst as a symbol of purity, and amethyst rings were given to bishops. Amethyst is also reportedly one of the original stones of the twelve tribes of Israel on Aaron's breastplate. Brazil is the best known source for amethyst, but deposits are also found in Siberia and North America. The characteristic purple color is due to traces of iron, and amethyst can be heat-treated to create citrine.

•• a m e t r i n e

As the name suggests, ametrine is a stone with naturally occurring amethyst and citrine. Both amethyst and citrine are varieties of quartz colored by iron or iron oxide, so the transition between amethyst and citrine can be a subtle one leading to beautiful stones.

•• a v e n t u r i n e

Aventurine is a type of quartz spotted or spangled in appearance due to inclusions of hematite (red aventurine), green fuchsite mica (green aventurine) or other minerals. When these inclusions are uniformly distributed and oriented, aventurine shows nice sparkle from the internal reflections. It's always found in massive form and has no cleavage planes so it's often used in cabachons.

•• b l o o d s t o n e 

Bloodstone is a dark fine-grained quartz, usually green, with patches of bright red jasper. India was the source for ancient gemstones, used for protection and health for the wearer. This March birthstone is also known as heliotrope, meaning "to turn the sun," referring to the belief that they could turn the sun red.

•• c a r n e l i a n

Iron oxide is responsible for the deep red-orange color of carnelian, a variety of chalcedony. It can be banded like agate or uniform in color and it can be light orange to deep blood-red in color. Like amethyst, it has a long history, used by Egyptians, Greeks and Romans, and it's also thought to be one of the twelve gemstones on Aaron's breastplate. India produces fine quality carnelian, as well as Scotland, Brazil and Washington. Natural carnelian is rare and therefore quite expensive, so most jewelry components are actually agate that has been heat treated and/or dyed.

•• c h a l c e d o n y

Chalcedony is a compact microcrystalline quartz that forms in cavities and cracks as silica-rich waters percolate through existing structures. Naturally it's pale in color but relatively porous, which works well for dyes. Most chalcedony jewelry components are dyed and subject to fading with time and exposure to sun.

•• c h a m p a g n e   q u a r t z

Champagne quartz is a pale, smooth, golden colored stone. It is a macrocrystalline quartz found in similar areas to crystal quartz and smoky quartz.

•• c h r y s o p r a s e

Chrysoprase is a variety of chalcedony (microcrystalline quartz) with distinctive apple-green to blue-green color from trace amounts of nickel. It is the most valuable of the chalcedonies and most fine quality stones come from Queensland, Australia. The Ural mountains in Russia, Brazil and California are other sources. It is generally untreated and the color may fade with exposure to sun.

•• c i t r i n e

Citrine is a yellow to yellow-orange color of macrocrystalline quartz. Hydrous iron oxide is responsible for the color, and many citrine components on the market are actually heated amethyst, which has similar composition. Transitions between amethyst and citrine occur naturally and stones showing this are called ametrine. Citrine is commonly heated to enhance color and sometimes dyed as well. High quality stones come from Scotland, Russia, France, Brazil and North Carolina.

•• c r y s t a l   q u a r t z

Crystal quartz or rock crystal is a colorless variety of macrocrystalline quartz. Quartz occurs in all kinds of silica-rich rocks (metamorphic, sedimentary, igneous) so the mineral itself is common, though finding flawless icelike specimens is not. It has been carved and used since ancient times; the name "quartz" is from Old German and crystal is derived from the Greek "krystalos".

•• g r e e n   q u a r t z

Green quartz or prasolite is sometimes sold as green amethyst, which is a misnomer since by definition, amethyst is purple quartz. However, it is fair to say that green quartz comes from amethyst. When natural amethyst is heated - either in a lab or by proximity to molten lava - it can produce prasiolite (green quartz). Prasiolite can also be created through radiation, though the color of irradiated green quartz tends to be less stable than heated green quartz. Green quartz is a pale fern or forest green and transparent, like the parent amethyst.

•• j a s p e r

Jasper is an opaque microcrystalline quartz that forms via deposition from silica-rich waters, like agate and other chalcedonies. There are numerous types of jasper, such as moukaite jasper from Australia, colored with clay and iron oxide, picasso jasper from Utah with striking patterns of black with gray, brecciated jasper that's a deep brick red from iron oxides and organic material, autumn jasper with swirls of red-brown and green, and fancy jasper that is a mix of muted jewel-tone hues.

•• l e m o n   q u a r t z 

Although lemon quartz is naturally quartz, the color is manufactured. The lemon bright color results from introducing iron to amethyst or yellow quartz at high heat. The treatment is permanent, and the gemstone maintains the hardness and durability of all quartz.

•• o n y x

Althought most onyx jewelry components are solid black, the definition of onyx is a striped variety of agate with white and black alternating bands. In fact, most "onyx" is agate that has been dyed black since uniform deep black is unusual to find in a mineral, other than schorl (black tourmaline) or jet. India and South America are the primary sources of natural onyx.

•• q u a r t z

This is a general category of quartz stones such as snow quartz, milky quartz or fossilized silicates like ammonites or petrified wood that don't fall into a different category.

•• r o s e   q u a r t z

Rose quartz is a translucent or transparent pink crystalline quartz. It is more commonly found in aggregate rather than prismatic form, and it may be transparent or cloudy due to minute rutile inclusions. Like amethyst and other macrocrystalline quartz, it is found in pegmatites. The primary sources are Madagascar, Brazil, Namibia, Sweden, Russia, Spain and the US states of California and Maine. It is often dyed to enhance or deepen the pink coloring.

•• r u t i l a t e d   q u a r t z

Rutile is a form of titanium oxide and rutilated quartz a variety of quartz that includes needle-like rutile within it. The quartz is typically transparent with various density of rutile needles; there may be few to so dense that the stone appears opaque in areas. The rutile is typically golden in color but can also be deep red to black. Black tourmaline may behave like rutile and show black needle-like inclusions; this is called tourmalinated quartz and may be difficult to distinguish from rutilated quartz.

•• s a r d o n y x

Sardonyx is onyx (variety of striped agate) though with the red-orange sard mineral instead of dark brown or black of onyx. Sard is similar to carnelian, though it tends to be slightly harder and darker red to red-brown.

•• s m o k y   q u a r t z

Smoky quartz is the brown color variety of macrocrystalline quartz whose color, like amethyst, is caused by the interaction of radiation with aluminum impurities within the crystalline structure. Smoky quartz is abundant, and it is possible to find jewelry components of excellent color and clarity at relatively inexpensive cost. Smoky quartz may be heated or irradiated to improve the color from grey-brown to a rich mocha color that is more popular.

•• s t r a w b e r r y   q u a r t z 

Strawberry quartz is a clear or colorless quartz with pale pink to dark pink mineral inclusions. The inclusions can be small platelets of hematite, flakes of lepidolite, flakes of alurgite (a orangy red muscovite), crystals of piemontite, or other minerals that are pink to red in color.

•• t i g e r s   e y e

Tiger's eye quartz is formed when parallel veins of crocidolite (blue asbestos) fibers are first altered to iron oxides then covered by silica. This leads to chatoyancy, a luminescent band appearing across the stone. The iron oxide gives tiger's eye a rich yellow to brown color. Blue tiger's eye is a natural stone in which silica covers the crocidolite directly, maintaining the blue-grey color. It should be noted that the asbestos fibers in the stones area altered or covered by other materials, so there is no risk of exposure. Tiger's eye comes from South Africa and Queensland, Australia.

••  m a t e r i a l   t y p e   l i n k s

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