Natural, untreated turquoise of good colour and hardness is scarce and expensive. The demand for turquoise-blue beads, however, is enormous and price-sensitive. The gap is filled three ways: by white minerals dyed blue, by low-grade turquoise impregnated with resin, and by turquoise powder pressed into blocks. A laboratory examination sorts them, and the sorting matters because the four categories differ in price by orders of magnitude.
What turquoise is
A hydrated copper aluminium phosphate. Properties: RI about 1.61–1.65 (aggregate; spot reading), SG 2.6–2.9 (lower if porous or treated), hardness 5–6, opaque, colour sky blue to greenish blue (iron gives green; copper gives blue), often with dark "matrix" veining of the host rock. Natural turquoise is porous; untreated material darkens with skin oils over years.
What is sold as turquoise
Dyed howlite. Howlite is a white mineral with grey veining, cheap and easily dyed. Dyed blue, its veins look like turquoise matrix. Tells: colour too uniform; dye concentrated at the drill hole and in surface pits; SG 2.5–2.6; hardness 3.5 (soft — a steel point marks it, though the laboratory does not scratch); the veining is grey rather than brown or black and often looks "painted on"; a swab of acetone may lift dye.
Dyed magnesite. Similar to howlite, white with veining, dyed blue; often sold as "turquoise magnesite" or simply as turquoise. SG 3.0, hardness 3.5–4.5; effervesces in acid.
Dyed marble, dyed calcite, dyed chalcedony. Less common; each with its own properties.
Plastic and resin. With painted or moulded matrix; warm, light, soft, may show mould seams; fluorescence.
Glass. Uncommon for turquoise imitation; bubbles.
"Block" turquoise, reconstituted turquoise. Turquoise powder (or unrelated powder) bound with resin, dyed, pressed and cut. Uniform texture with no natural matrix pattern; resin fluorescence; resin visible at magnification as a binder between grains. Often sold as turquoise without qualification.
Treatments of genuine turquoise
Stabilised. Porous, chalky turquoise impregnated with resin or wax under pressure to harden it and deepen the colour. Very common — most turquoise in jewellery is stabilised. The stone is genuine turquoise; the treatment is standard and should be disclosed. Detected by resin fluorescence, the "plastic" surface feel and appearance under magnification, and sometimes by a solvent test.
Colour-enhanced (dyed) turquoise. Genuine but pale turquoise dyed to deepen the blue, often combined with stabilisation.
Waxed. Surface wax to improve lustre; minor.
"Zachery" treatment. A proprietary treatment that improves hardness and colour without resin; detectable only by advanced analysis (potassium content).
How the laboratory examines a claimed turquoise
Magnification for matrix character, dye concentration in pits and drill holes, and resin binder. Specific gravity. UV for resin fluorescence. Spectroscope where useful (natural turquoise shows a copper-related absorption). Hardness inferred from identity. Report states: natural turquoise (and whether stabilised or dyed), reconstituted, or dyed howlite/magnesite/other.
Frequently asked questions
Is stabilised turquoise fake?
No. It is genuine turquoise that has been hardened with resin. It is inferior to untreated natural turquoise and should be sold as stabilised.
How can I spot dyed howlite at home?
Look at the drill hole and any chips under a loupe: dye concentrated there, with white beneath, is the sign. Also, howlite veining is grey and uniform; turquoise matrix is brown to black and irregular.
Does natural turquoise change colour?
Untreated turquoise absorbs skin oils and can turn greener over years. Stabilised turquoise does not.
What is "sleeping beauty" turquoise?
A trade name for clean, matrix-free blue turquoise from a particular Arizona mine, usually stabilised. Widely imitated by dyed material.
Is reconstituted turquoise disclosed?
It should be, and often is not. A uniform blue bead with a "granular" look and no natural matrix pattern is a candidate.