Chalcedony is a cryptocrystalline form of silica, composed of very fine intergrowths of the minerals quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic. Chalcedony's standard chemical structure (based on the chemical structure of quartz) is SiO2 (Silicon Dioxide).
Chalcedony has a waxy luster, and may be semitransparent or translucent. It can assume a wide range of colors, but those most commonly seen are white to gray, grayish-blue or a shade of brown ranging from pale to nearly black.
en.wikipedia.org/wiki/Chalcedony Chalcedony Pictures
Colors / Varieties:
Agate (curved bands)
Fire Agate (iridescence)
Moss Agate (dendritic)
Onyx (black & white bands, generally straight)
Sardonyx (brown & white bands)
Jasper: brownish red, yellow, black, may be variegated colors, banged (ribbon jasper), black fine grained (touch stone or bassanite, used for testing gold alloys).
Milky chalcedony (may show a blue sheen)
Carnelian (red, orange, yellow)
Prase (yellow - green)
Chrysoprase (apple green - yellow green)
Blood Stone / Heliotrope (green with red spots)
Chrome Chalcedony (green)
Sard (brown, brownish red)
Impregnated chalcedony (e.g. white chalcedony full of red cinnabar will appear wholly red is known as myrickite)
Crystal System / Forms:
Trigonal System / Cryptocrystalline aggregates, often in botryoidal form.
2.58 - 2.62
Cleavage / Fracture:
None / Conchoidal fracture with a dull or waxy lustre.
Vitreous to waxy.
Refractive Index / Birefringence:
1.539 - 1.543 (Spot
1.530) / 0.004.
Flow structures (white / colored concretions, botryoidal as in fire agate), banding (straight / curved), dendritic / crystal inclusions.
nm - 700 nm. Chrome chalcedony: lines in red.
Dyed green chalcedony: band at 650
Cause of Color:
Chrysoprase: Ni - rich clay inclusions.
Chrome chalcedony: Cr
Yellow, orange, brown, red: Fe
Irridescence: Interference from fine layers.
Simulants (with separation tests):
(hardness, lustre), Onyx Marble (birefringence),
, structure), etc.
In varied deposits, as veins, cavity fillings, etc.
Brazil, India, Russia, Iceland,
, Germany, etc.
Cuts & Uses:
Cabochon, beads, cameos, intaglios, carvings, tablets.