The word turquoise means “Turkish” in French.

This stone has little to do with Turkey. It is found primarily in Iran, China, the American Southwest, and Egypt’s Sinai Peninsula. Turkey has minor turquoise deposits, but the output is small and the quality unremarkable. The reason Europeans named it after Turkey is that the stone entered Europe through Turkey — medieval Genoese and Venetian merchants bought it in the markets of Constantinople, assumed it came from there, and the name stuck.

The largest copper-bearing mineral actually produced in Turkey is malachite. The stone named after Turkey comes from somewhere else entirely.

An incorrect name, in circulation for several hundred years, now impossible to change.

The most important source of turquoise is in northeastern Iran, near the ancient city of Nishapur.

Nishapur was a major node on the Silk Road, once the capital of the Khorasan region. Its turquoise mines have been worked for more than two thousand years; the earliest references in Persian literature date to before the common era. The mining area lies in the Ali-Mersagh mountains to the northwest of the city, at an elevation of approximately two thousand metres, the veins running through weathered trachyte.

Nishapur turquoise is known for its colour. The finest material is a pure sky blue that Persians call Abi Gai — water of the sky. This colour comes from copper: turquoise is a hydrated copper aluminium phosphate, and the ratio of copper to iron determines whether the stone reads blue or green. Nishapur’s veins are high in copper and low in iron, which produces a blue that is clean and without green undertone.

The mines are still operating, but output has declined sharply. The best veins were largely exhausted decades ago. Material now labelled Nishapur on the market comes from there in diminishing proportion.

Turquoise is a porous mineral. This is its physical nature, and the source of the most persistent debate about it in the trade.

The pores absorb oils, perspiration, and environmental chemicals. The colour shifts over time — a process collectors call patination. Some regard this as evidence that the material is alive; others regard it as degradation. To stabilise the colour, most turquoise on the market is treated with wax or resin, filling the pores and preventing change. Wax treatment is considered acceptable within the industry. Resin treatment is more contested, because it alters the physical structure of the material. Untreated natural turquoise changes slowly, in response to the skin oils and habits of whoever wears it. Each piece changes differently.

The Persians had a way of describing this: the colour deepens because the stone is absorbing the breath of its wearer. There is no scientific basis for this. The physical process it describes is real.