Chalcedony

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.

Source: en.wikipedia.org/wiki/Chalcedony

Chalcedony Pictures

Onyx (a variety of chalcedony) swan

Chalcedony Properties

Chemical Composition: 
Silica SiO2
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.
Hardness: 
6.5
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.
Pleochroism: 
None.
Dispersion: 
None.
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): 
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.