Turquoise

What it actually is

Hydrated copper aluminium phosphate, CuAl6(PO4)4(OH)8 4H2O, triclinic, hardness 5 to 6, and porous.

Copper gives the blue; iron substituting for aluminium shifts it towards green. That is why turquoise runs from sky blue through to yellowish green depending on the deposit's chemistry, and why individual mines are recognisable by colour and matrix.

Porosity is the defining practical property. Turquoise absorbs oils, cosmetics, sweat, sunscreen and cleaning products, all of which shift the colour towards green permanently. Older traditions read that change as the stone taking on the wearer's life, which is a graceful description of a chemical process.

Matrix is the host rock left in the stone, appearing as brown, tan or black veining. Spiderweb turquoise has a fine even network of it and is prized rather than penalised.

Most commercial turquoise is stabilised, meaning impregnated with resin to make soft porous material workable and colour-stable.

Where it comes from

Iran's Nishapur deposits have been worked for millennia and produced the benchmark sky-blue material sometimes called Persian turquoise.

Egypt worked Serabit el-Khadim and Wadi Maghareh in the Sinai from before 3000 BCE under state expeditions, recorded on stelae at the temple of Hathor, who was titled the lady of turquoise.

The American Southwest has worked deposits including Cerrillos in New Mexico for around two thousand years, and Cerrillos turquoise travelled over a thousand miles into Mesoamerica as a trade good. Named American mines such as Sleeping Beauty, Bisbee, Kingman and Number Eight command premiums and their material is recognisable to specialists.

China is now the largest producer by volume, and Mexico, Chile and Australia also supply it.

The English name records the trade route: turquoise means Turkish, because Persian material reached Europe through Turkish traders.

What the traditions claimed

The traveller's association is the most consistent stone meaning found anywhere. Turquoise on a horse's bridle to prevent falls appears in Persian and Turkish practice; turquoise on riders appears in the American Southwest; and safe-passage meanings recur around the Mediterranean.

Egyptian use tied it to Hathor and to the colour vocabulary of renewal, and it appears in funerary jewellery including the mask of Tutankhamun.

In Pueblo, Navajo and Zuni practice turquoise carries a significance that is ongoing rather than historical, connected with water, sky and ceremony. It should be described as a living tradition, and material and designs from those communities are subject to real concerns about appropriation and misrepresentation.

Persian tradition held that turquoise changing colour warned of danger or illness, which is an interpretation of the porosity effect.

Modern practice reads it as a stone of protection, communication and healing, which is broadly continuous with the older travel and protection readings.

How it is worked with

The travel reading is the one to use, and it is unusually well supported. A turquoise carried or worn at the start of a journey is doing what the stone has done on three continents.

Care is where most people fail it. Keep it away from perfume, hand cream, sunscreen, detergent and cleaning products. Put it on after cosmetics and take it off before washing up. Never soak it, never use an ultrasonic cleaner, and keep it out of high heat, which can crack it and damage stabilising resin.

Expect it to change colour over years if worn against skin. Decide in advance whether you regard that as damage or as a record.

Buy from someone who will name the mine. That single question filters out most of the problems described below.

Buying it honestly

This is the worst market in the catalogue.

A large majority of what is sold as turquoise is not. Dyed howlite and dyed magnesite are the commonest substitutes, both around hardness 3.5 where turquoise is 5 to 6, and both showing grey-to-black veining that dye has coloured around rather than genuine brown or tan matrix. Acetone on a cotton bud lifts the dye.

Reconstituted turquoise is genuine powder bound in resin and looks uniformly grainy under a loupe.

Block turquoise is entirely synthetic plastic or resin.

Stabilised turquoise is genuine turquoise impregnated with resin, which is normal, standard and should be disclosed. It is not the same as fake.

Price is the strongest signal. Natural untreated high-grade turquoise is expensive and always has been. A large bright blue piece for a few pounds is not it.