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Chalcedony

chalcedony
Brown chalcedony rough

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

Cause of Color :
  • Chrysoprase: Ni – rich clay inclusions.
  • Chrome chalcedony: Cr
  • Yellow, orange, brown, red: Fe
  • Irridescence: Interference from fine layers.

Chemical Composition :

Silica SiO2


Crystal System / Forms :

Trigonal System / Cryptocrystalline aggregates, often in botryoidal form.


Cuts & Uses :

Cabochon, beads, cameos, intaglios, carvings, tablets.


Dispersion : None.
Hardness : 6.5
Lustre : Vitreous to waxy.
Magnification :

Flow structures (white / colored concretions, botryoidal as in fire agate), banding (straight / curved), dendritic / crystal inclusions.


Optic Character :

Aggregate (A.G.G.)


Pleochroism : None.
Refractive Index / Birefringence :

1.539 – 1.543 (Spot R.I. 1.530) / 0.004.


Simulants (with separation tests) :

Feldspar (structure, phenomena), Serpentine (hardness, lustre), Onyx Marble (birefringence), Amber (S.G., structure), Jade (R.I., S.G., structure), etc.


Sources :

Brazil, India, Russia, Iceland, U.S.A., Germany, etc.


Spectrum :
  • Chrysoprase: 660nm – 700nm.
  • Chrome chalcedony: lines in red.
  • Dyed green chalcedony: band at 650nm

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