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.
Chemical Composition:
Silica SiO
2
Colors / Varieties:
Banded
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).
Unbanded (various colors):
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)
Plasma (green)
Blue chalcedony
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.
Specific Gravity:
2.58 - 2.62
Cleavage / Fracture:
None / Conchoidal fracture with a dull or waxy lustre.
Optic Character:
Aggregate (
A.G.G. )
Lustre:
Vitreous to waxy.
Refractive Index / Birefringence:
1.539 - 1.543 (Spot
R.I. 1.530) / 0.004.
Magnification:
Flow structures (white / colored concretions,
botryoidal as in fire agate), banding (straight / curved), dendritic /
crystal inclusions.
U.V. Fluorescence:
Inert.
Spectrum:
Chrysoprase: 660nm - 700nm .
Chrome chalcedony: lines in red.
Dyed green chalcedony: band at 650nm
Cause of Color:
Chrysoprase: Ni - rich clay inclusions.
Chrome chalcedony: Cr
Yellow, orange, brown, red: Fe
Irridescence: Interference from fine layers.
Treatment (Enhancement):
Coating
Colorless Impregnation
Colored Impregnation (Dyeing) - Dyed in almost all colors. Sold as black / green / yellow onyx etc.
Heat treatment (orange / red to brown).
Simulants (with separation tests):
Feldspar (structure, phenomena), Serpentine (hardness, lustre), Onyx Marble (birefringence), Amber (
S.G. , structure), Jade (
R.I. ,
S.G. , structure), etc.
Geological Occurrence:
In varied deposits, as veins, cavity fillings, etc.
Sources:
Brazil, India, Russia, Iceland,
U.S.A. , Germany, etc.
Cuts & Uses:
Cabochon, beads, cameos, intaglios, carvings, tablets.
