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Different Varieties of Agate Gemstones

Distinguished in most cases by strong bands of color, agate is a form of chalcedony, a variety of fine quartz. It forms over time in air pockets in igneous rock, taking its color from the impurities that crystalize there. When used in jewelry, it is cut "en cabochon" -- that is, with one flat and one rounded surface. There are several varieties of agate, differentiated by their physical properties.
  1. Onyx

    • Found in India and South America, Onyx is a form of agate with distinct black and white stripes. There is an abundance of artificially stained fake onyx on the market, but the genuine article is quite rare. In jewelry, it was used in the 18th century as a prestigious material for cameos, miniature carvings where the foreground and background are in contrasting colors. A similar agate also employed for cameos is sardonyx, which has brown and white layers.

    Fortification Agate

    • This type of agate is so named because, when it is sliced, the banding resembles the ground plan of a castle. Within this broad category, there are varieties distinguished by their coloring, pattern and place of origin. Blue lace agate from South Africa has a very pale, delicate pattern. Brazilian agate has a pattern of circles like the defenses of a Medieval keep. Crazy lace, from Mexico, has an almost psychedelic coloring and pattern.

    Moss Agate

    • Very different from all of the other varieties of agate in that it doesn't have any banding, moss agate takes its name from the "dentritic" -- tree-like -- inclusions it contains. These look remarkably like preserved bits of ancient fern, but are in fact mineral deposits. Sources of moss agate include India and Wyoming in the United States. When used in jewelry, moss agate is valued by collectors, so it is well worth snapping up any antique examples you may find.

    Tiger's-Eye

    • Popular as a gem for pendants, this variety of quartz is often confused with agate, so it is worth mentioning here. Found in South America and South Africa, tiger's-eye is a brown stone that exhibits a quality known as chatoyancy. This means that when you tilt the stone, a luminous band runs across it vertically. The band resembles a cat's pupil, hence the name. There is also a related stone called hawk's-eye, which has a bluish tinge.


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