Turquoise.

Turquoise is a hydrated copper aluminium phosphate, formed where copper-rich groundwater works through aluminous host rock in arid climates. The colour comes from copper, with iron substitution shifting the tone toward green. Most natural turquoise is porous and relatively soft (five to six on Mohs), which is why so many pieces in the trade are stabilised with resin to prevent colour change with skin oils and moisture.

The Persian deposits at Nishapur, worked since at least the second millennium BCE, set the historical reference for the pure sky-blue tone often called Persian or robin's-egg blue. Southwest American deposits, especially the Sleeping Beauty, Bisbee, and Lone Mountain mines, produced the matrixed material that defines Navajo, Zuni, and Pueblo silverwork. Egyptian Sinai turquoise appeared in funerary jewellery from the earliest dynasties.

Across all of these traditions the meaning has stayed close to the same. Turquoise is a stone of safe travel, of honest speech, and of protection in the practical sense (warding off the small daily harms rather than the great catastrophes). Modern crystal practice pairs it with the throat chakra. Care is straightforward but not optional: keep turquoise away from cosmetics, perfume, prolonged water contact, and direct sun. Soap residues will slowly discolour even good stabilised pieces. A soft dry cloth is enough.