CHALCEDONY
Silicon dioxide, better known as quartz, is one of the most abundant minerals on Earth. Mineralogists divide quartz into two classifications: crystalline and cryptocrystalline, based on the size of its crystals. Cryptocrystalline is also called microcrystalline quartz.
Due to its distinct crystal formation, crystalline quartz is usually transparent. Rock crystal, amethyst, citrine, and rose quartz are well-known examples of crystalline quartz. (See Facets of Quartz: http://www.suite101.com/article.cfm/gems...)
The atoms in cryptocrystalline quartz pack together to form stones that are either opaque or translucent. The atoms take the form of fibers, rather than crystals, and the stones often contain water or air trapped between the layers of fibers.
The cryptocrystalline quartzes are informally divided into two groups: jasper and chalcedony (kal-ced-nee). The jaspers are often opaque, while the chalcedonies tend to be transparent. Most mineralogists don't distinguish between the two, simply referring to both as cryptocrystalline.
Because there are too many cryptocrystalline quartzes to describe in one article, we'll use the commonly accepted groupings. Onyx, prase, sard, sardonyx, and tiger's eye are usually classified as jaspers. The chalcedony family generally includes agate, aventurine, bloodstone, carnelian, chrysocolla, and chrysoprase.
As described above, chalcedony is a group name. It also is the name of a particular stone within that group. Blue Mist chalcedony is a pale, almost white shade of blue that glows beautifully. It's durable and hard, making it a good choice for jewelry.
Agate probably has the greatest variety of color and variety of any gemstone. It's also one of the most abundant of all stones. Named after the Achates River, which flowed through Sicily in ancient times, agate takes a brilliant polish and has been widely used. Archeologists have found agates used by humankind more than 20,000 years ago.
Most mineralogists require agates to show banding (stripes), however many kinds of chalcedony without banding are commonly referred to as agates. Dentritic agate, which appears to have ferns or tiny trees enclosed within it, is one example. Moss agate and plume agate are also unbanded chalcedony.
Ancient peoples ascribed both magical and medicinal powers to agate. Placing an agate in the mouth relieved thirst, placing it against the forehead relieved a fever, and wearing an agate helped its possessor speak only the truth. Agate was also carved, especially into bowls. European museums have extensive collections of these bowls. Egyptians also carved agates into cameos more than 3000 years ago.
Aventurine is another chalcedony that carvers used
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