Chalcedony Cryptocrystalline Quartz
Chalcedony which it's generally agreed should be pronounced kal~SED~uh~nee, is a fine grained or cryptocrystalline variety of quartz. Microcrystalline and cryptocrystalline are both words which tend to be used interchangeably to describe minerals whose crystals are too small to be seen without the aid of magnification. When the mineral quartz is either microcrystalline or cryptocrystalline it's correctly known as chalcedony. Jasper is a common variety of chalcedony whilst others include agate, carnelian, onyx, chrysoprase, and bloodstone. Chalcedony which is usually translucent is most commonly found in white, grey, blue or brown although it can exhibit shades of many other colours due to the presence of other minerals although often they'll only be microscopic.
The colour of carnelian is caused by the presence of iron oxides which are chemical compounds made up of iron and oxygen, the green of chrysoprase is caused by nickel oxide whilst bloodstone features inclusions of red jasper. Some of the lesser known varieties of chalcedony include plasma, sard and sardonyx. Sard is translucent and occurs in shades of light to dark brown and sardonyx features bands of sard in addition to white chalcedony. Stones that feature colourful bands which are particularly distinctive are mostly referred to as agate but when the bands are black, white or dark brown, the stone becomes known as onyx. Chalcedony usually has a waxy lustre.
Chalcedony is known to have been used by man for thousands of years and is thought to have been one of the earliest materials used for tools and weapons. Agate, flint, jasper and petrified wood were also widely used and all four are varieties of chalcedony.
Considered to be a sacred stone by the Native American Indians, chalcedony was used to promote stability within their tribes. In ancient times it was used to make goblets lined with silver which were used in the belief that they would offer protection against poisoning. Today in crystal healing chalcedony is said to encourage generosity, to protect against nightmares, fear of the dark, hysteria, depression and also to combat negative thoughts.
Graded 6½ to 7 on Mohs scale of mineral hardness, chalcedony is found pretty much worldwide although the finest grade stones generally come from India, Madagascar, Burma, Brazil, Mexico and the USA.
Wikipedia on Chalcedony