Turquoise is a hydrated copper aluminium phosphate, and it is among the very oldest gems humans have mined: Egyptian expeditions worked the Sinai deposits over 6,000 years ago, and it has deep significance in Persian and Native American cultures alike. Copper gives the blue, and iron or zinc shift it toward green; the famous even sky blue of Nishapur in Iran set the standard, while American classics come from mines like Sleeping Beauty and Kingman in Arizona. Spider-web veining of the host rock, called matrix, is prized in its own right. It is a birthstone for December.
Turquoise is somewhat soft, at 5 to 6 on the Mohs scale, and naturally porous, so perfume, oils and cleaning sprays can discolour it over time. Put it on last and wipe it with a soft, dry cloth. Most affordable turquoise is stabilised with resin to harden it and hold its colour, a legitimate, disclosed treatment; untreated natural stone costs considerably more. Dyed howlite and plastic are the common fakes, so buy from a seller who states exactly what you are getting, as we always do.
Variscite is its greener phosphate cousin, and chrysocolla and larimar share the sky-and-sea palette.
