Phosphate
Hydrated copper aluminium phosphate, prized for six thousand years across Persia, Egypt and the American Southwest. Color ranges from Persian sky blue to Sonoran green, often webbed with matrix.
Most turquoise on the market is stabilised with resin, which is acceptable and disclosed; dyed howlite and 'block' plastic turquoise are not. Natural high-grade material is dense, takes a waxy polish, and shows consistent color through a chip.
Porous and reactive: keep away from oils, perfume and household chemicals, all of which turn it green.
Nishapur in Iran has produced turquoise continuously for millennia and gave the world the reference standard 'Persian blue'.
Cultural history, recorded as tradition rather than as medical or scientific claim.
See it in person
Both rooms are open six days a week, and there is always someone happy to pull pieces out of the case so you can compare them under the light.