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.

Chemical Composition: 

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

Colors / Varieties: 

• Opaque blue, blue - green, greenish - blue, yellowish - green. Green (increasing Fe %)
• Turquoise matrix: contains parts of the mother rock.
• Spiderweb turquoise: matrix resembling a spider web pattern.

Crystal System / Forms: 

Triclinic System / Cryptocrystalline aggregates, massive, varying porosity.

Hardness: 

5.5 - 6

Specific Gravity: 

Range: 2.60 - 2.85 (varies with porosity)

Cleavage / Fracture: 

None / Conchoidal fracture with a dull to waxy lustre.

Optic Character: 

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

Lustre: 

Polished surfaces are waxy to vitreous.

Refractive Index / Birefringence: 

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

Pleochroism: 

None.

Magnification: 

Matrix background may contain blackish metallic inclusions.

U.V. Fluorescence: 

Greenish yellow to blue in longwave and inert in shortwave.

Spectrum: 

Weak bands at 460nm, line at 432nm

Cause of Color: 

Blue - copper; green - copper and iron.

Treatment (Enhancement): 

• Colorless impregnation: Use of plastic, paraffin, waxes to seals the pores and improves durability and surface lustre. Surface coating may be used.
• Colored impregnation (dyeing): Use of color / yellow - brown metallic material to improve color and / or introduce a matrix effect.

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.

Synthesis: 

Gilson synthetic turquoise by the ceramic method.

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.

Geological Occurrence: 

In areas of copper deposits, in porphyry and trachyte rocks.

Sources: 

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

Cuts & Uses: 

Cabochon, beads, carvings, tumbled, etc.