Turquoise

turquoise
Turquoise rough

One of the first gemstones to be mined, turquoise has long been prized for its intense color, which varies from sky-blue to green, depending on the quantities of iron and copper within it. Turquoise is commonly found in microcrystalline, massive form, usually as encrustations, in veins, or as nodules. It is opaque to semi-translucent, light and very fragile, with conchoidal fracture. Some material is very porous, leading to fading and cracking, so it may be impregnated with wax or resin to maintain its appearance.

Sky blue turquoise from Iran is generally regarded as the most desirable; but in Tibet a greener variety is preferred. Localities in Mexico and United States produce a greener, more porous material that tends to fade more quickly. Other localities include the former USSR, Chile, Australia, Turkistan and Cornwall (England).

Turquoise has been thought to warn the wearer of danger or illness by changing color. It has been imitated by stained howlite, fossil bone or tooth, limestone, chalcedony, glass, and enamel. In 1972, an imitation turquoise was produced in France by Gilson.

Turquoise Pictures

Different types and styles of turquoiseTurquoise beadsTumbled turquoise

Cause of Color :

Blue – copper; green – copper and iron.

Chemical Composition :

Hydrous copper aluminium phosphate, with traces of Iron. CuAl6(PO4)4(OH)8.5H2O

Crystal System / Forms :

Triclinic System / Cryptocrystalline aggregates, massive, varying porosity.

Cuts & Uses :

Cabochon, beads, carvings, tumbled, etc.

Hardness : 5.5 - 6
Lustre : Polished surfaces are waxy to vitreous.
Magnification :

Matrix background may contain blackish metallic inclusions.

Optic Character :

Anisotropic, D.R.; Biaxial positive. Aggregate (A.G.G. to Opaque).

Pleochroism : None.
Refractive Index / Birefringence :

1.610 – 1.650 (Spot R.I. 1.610) / 0.040.

Simulants (with separation tests) :
  • Synthetic turquoise: regular surface texture with blue spheres in a white background, blue thread like inclusions.
  • Reconstructed turquoise (plastic / resin bonded): Hot point test will give typical odor.
  • Variscite (R.I., structure, spectrum), Chrysocolla (R.I., S.G., texture), Plastic (S.G., structure), Glass (lustre, inclusions), Pectolite (structure, spectrum, birefringence), amazonite (structure, R.I., S.G.), Dyed Blue Howlite (spectrum, structure) etc.
Sources :

Iran (least porous), U.S.A. (Arizona, New Mexico), Tibet, Mexico, Egypt, Australia, China

Specific Tests :
  • Because of porosity, should be treated with care. Body oils and perfumes may be absorbed and cause splotches of discoloration, and also a change of color.
  • Dissolves slowly in hydrochloric acid.
Spectrum :

Weak bands at 460nm, line at 432nm

Synthesis :

Gilson synthetic turquoise by the ceramic method.

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