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www.mineralgallery.co.za Minerals A-E |
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Chalcedony |
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Name
Chalcedony
Chemistry
SiO2
Uses
As a specimen
Color
The whole spectrum of colors. Agate a variety of chalcedony can have more than one color band in the same stone.
Hardness
7
Specific gravity
2.6-2.75
Chalcedony is a fine-grained, microcrystalline variety of quartz that is similar to chert and flint. Chalcedony is composed of silicon dioxide and occurs in rounded masses with a waxy luster, splintery fracture, and fibrous structure. It is usually brown to gray in color and some varieties are often noted for their bands of color.
Used for centuries as the principal engraved and carved ornamental stone, chalcedony is also rounded and polished as a semiprecious gem. Agate, bloodstone, carnelian, onyx, and sardonyx are semiprecious varieties of chalcedony. Some varieties are florescent. The fracture is conchoidal.
These are the main varieties:
Agate - Banded variety
Carnelian - Reddish, transparent to translucent variety
Onyx - Banded variety in which the banding lines are straight and parallel, and consistent in band size.
Jasper - Opaque variety of Chalcedony that occurs in all colors.
Tiger's Eye - Pseudomorph of Quartz after fibrous Crocidolite.
Chrysoprase - Apple-green variety
Bloodstone (or Heliotrope) - Dark green to greenish blue variety dotted with small, red, bloodlike spots.
Sard - Brownish to brownish-red, transparent to translucent variety
Sardonyx - Banded variety with straight parallel bands of brownish to red alternating with white or black bands.
Flint - Massive, uniformly colored variety that is somewhat impure.
Most of these are only known by their popular name and not as chalcedony. Agates are usually unatractive before they have been cut and polished. When cut and polished they turn out very attractive. Sometimes they are coloured by staining them.
