Quartz is one of the most common minerals on earth and the quartz family is made up of two separate branches. When most people think of quartz, they think of what is known as macrocrystalline quartz, which includes rock crystal, amethyst, citrine, smoky quartz and rose quartz. Macrocrystalline quartz, as the name suggests, has large crystals which can be distinguished by the naked eye. This type of quartz is mainly transparent to translucent, with a vitreous luster.
The other type of quartz is known as cryptocrystalline quartz. It has microscopically (or submicroscopically) small crystals and is usually translucent to opaque, with a waxy to greasy or dull luster. This kind of quartz has fibrous and granular subcategories. The fibrous varieties are known under the name chalcedony, but this name covers a remarkable variety of stones.
In practice, the name chalcedony refers to solid colored cryptocrystalline quartz, especially of a light color. The patterned varieties tend to have their own names. The most famous of these include agate, which is distinguished by its bands of color; black and white layered material known as onyx (this is usually dyed to produce a uniform black); carnelian, which is yellow-orange to reddish-orange and is colored by iron; and chrysocolla, a rare type of vivid blue-green cryptocrystalline quartz that is colored by copper.
The patterns of agate are so distinctive that a number of different varieties are commonly identified in the trade. They include eye agate, which forms ring shapes with a point in the center; dendritic agate, which is distinguished by its moss-like patterns; and fire agate, a type of agate that has an unusual iridescence.
Agate was used as a gemstone by the Egyptians at least 3,000 years ago. In modern times, agate has a special association with the Idar-Oberstein region in Germany. Idar-Oberstein was an important source of agate until the 19th century. The region was favored with good local sandstone for producing cutting and polishing wheels, and water power to work the wheels.
Fortunately for Idar-Oberstein, large deposits of agate were discovered in Brazil in the 1830s, just as the agate mines in Germany were starting to be worked out. The agate polishing industry in Germany flourished as a result and produced high quality objets d'art, beads, rings, pendants, brooches and cameos.
Today agate continues to be mined in Brazil. It is also mined in Uruguay, India, Australia, China, Madagascar, Mexico, Mongolia and Namibia.
- First Published: January-27-2009
- Last Updated: June-24-2014
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