Chalcedony

Chalcedony

Chalcedony is a form of quartz which is cryptocrystalline. Microscopically small crystals of quartz are embedded in opal, which is itself hydrated quartz. The proportion of quartz to opal varies, and this variation is reflected in differing densities, refractive indexes and other properties. The quartz crystals are in a fibrous form, sometimes running approximately parallel to each other, sometimes entwined.Another name for chalcedony is agate, although the term agate is usually used to describe the banded varieties of chalcedony. Many agates are naturally coloured, others are stained for decorative effect. Varieties include chrysoprase, cornelian (carnelian, sard, sardonyx, onyx, plasma, bloodstone, myrickite. There are many differently named varieties of agate depending on their appearance such as moss agate, fossilised wood. and other forms of fossil. Jasper also belongs to this group, and contains impurities which cause heavy pigmentation.