Chalcedony
Chalcedony is a group of cryptocrystalline quartz aggregates with a large number of varieties. Also described as microcrystalline or polycrystalline quartz, it comprises a mass of minute crystals with a fine fibrous or grainy structure. The crystals are only visible when viewing very thin sections of material through a microscope, but are difficult to distinguish.
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:
Chalcedony
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'.
Chrome 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.
Onyx
Also called Layered Onyx, is a black and white stripped variety, used to make intaglios and cameos.
Black onyx is a single coloured variety.
Carnelian
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.
Chrysoprase
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.
Prase
A leek-green translucent variety of chalcedony, coloured by inclusions of chlorite and horneblende fibres. From the Greek word 'prason' meaning leek.
Heliotrope
A dark green stone with red spots, commonly called bloodstone.
Sard
A translucent to opaque chalcedony, occurring in various shades of brown and brownish-red.
Agate
A banded member of the group with many varieties. Agate may have bands in multiple colours or in shades of a single colour; some bands may contain opal. To be a true variety of agate, the stone must have bands and although some named agates have their own optical effect, they do not have bands.
Fire agate
Displays iridescent colours caused by thin films of iron between layers of chalcedony.
Fossilized wood
Wood that has been petrified (replaced) by minerals over many millions
of years. The wood is commonly replaced by jasper chalcedony, unless silicon dioxide was the only available mineral during the replacement; this would produce opalised wood.
Moss agate
A non-banded chalcedony with dendritic moss like inclusion of hornblende in a translucent near colourless material.
Dendritic agate
A non-banded chalcedony that shows tree-like patterns.
Scenic agate
A non-banded chalcedony with patterns that resemble landscapes.
Iris agate
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.
Properties of the Gemstone and Additional Information
| Gemstone type: | A group of cryptocrystalline quartz aggregates. Examples include: Agate, Chrysoprase, Carnelian, jasper, onyx, sard and chalcedony (by it's own name). |
| Crystal System: | Trigonal |
| Chemical Composition: | Silicon dioxide SiO2 |
| Colour: | The colour varies throughout the chalcedony group, as described above. |
| Lustre: | Dull or waxy |
| Pleochroism: | Not present |
| Dispersion: | None |
| Hardness: | 61/2 to 7 on the Mohs' scale |
| Toughness: | Very good |
| Cleavage: | None |
| Density in gm/cc: | 2.58 -2.64 |
| Double refraction: | 0.0004-0.009 |
| Refractive index: | 1.53-1.54 |
Common Treatments
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.