Chalcedony is a group of polycrystalline quartz aggregates with a large number of varieties. It comprises a mass of minute crystals with a fine fibrous or grainy structure.The name chalcedony is derived from the ancient Greek town of Chalkedon in Asia Minor, an area that is now part of modern day Istanbul. Chalcedony is also the name of a specific variety within the group.
Varieties of chalcedony include:
A translucent to opaque variety with colours ranging from white to grey to bluish. When blue it is sold at a premium and marketed as 'blue chalcedony'.
A green variety of chalcedony, coloured by chromium. Also called Mtorodite in Zimbabwe, named after the mountainous region of Mtoro where deposits are found.
Also called Layered Onyx, is a black and white stripped variety, used to make intaglios and cameos. Black onyx is a single coloured variety.
A red variety with colours ranging from flesh-red to red to brownish-red; it can be similar in colour to sard when brownish red. The name comes from the Latin for horn, due to a similarity in hue to the flesh-red stone. Treated carnelian is often a deeply saturated orangey-red.
An apple coloured variety of and the most valuable of the chalcedony group. The name is derived from the Greek words 'chrysos' meaning gold and 'prason' meaning leek. The colour, which varies from pale to vivid green, is unstable in sunlight or high temperatures. A chrysoprase matrix also occurs with brown or white markings.
A leek-green translucent variety of chalcedony, coloured by inclusions of chlorite and horneblende fibres. From the Greek word 'prason' meaning leek.
A dark green stone with red spots, commonly called bloodstone.
A translucent to opaque chalcedony, occurring in various shades of brown and brownish-red.
A banded member of the group with many varieties of it's own. Agate may have bands in multiple colours or in shades of a single colour; some bands may contain opal. Although described as a banded variety of chalcedony, agate also occurs without bands. Those varieties have their own optical effects as described below.
A non-banded chalcedony with dendritic moss like inclusion of hornblende in a translucent near colourless material.
A non-banded chalcedony that shows tree-like patterns.
A non-banded chalcedony with patterns that resemble landscapes.
An iridescent variety of agate that owes the effect to a structure similar to that of a diffraction grating. When cut into thin slices, transmitted light passing through the stone is dispersed into spectral colour. Quartz fibres in long parallel formations for the structure that causes the diffraction.
Displays iridescent colours caused by thin films of iron between the layers of chalcedony.
Wood that has been petrified, has been replaced by minerals over many millions of years. The wood is commonly replaced by jasper chalcedony. It may also be replaced by silicon dioxide, in which case it would have produced opalised wood.
Banded agate has porous layers that are dyed by boiling the finished article in chemicals.
Some varieties of chalcedony may be heat-treated to improve their colour and also to reduce the visibility of inclusions.