Chalcedony

Properties

Chemical Composition: Silica SiO₂

Classification / Type: -

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): 

• Coating
• Colorless Impregnation
• Colored Impregnation (Dyeing) - Dyed in almost all colors. Sold as black / green / yellow onyx etc.
• Heat treatment (orange / red to brown).

Specific Tests & Remarks: -

Synthesis: -

Simulants (with key 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.