of a light blue or blue-green color. The blue color comes from copper
, the green from iron
It's usually found in a matrix
of brown or gray rock, so that it is hard to find a gem-size chunk of turquoise without any streaks of other rock (often veins of black manganese
) running through it -- the more matrix a piece contains, the lower the price of that piece of turquoise. Robin's-egg blue is the most sought-after color.
Turquoise is rather porous, so it is often oiled or waxed after polishing to keep it from absorbing things from its environment (sometimes also to get it to absorb a dye). Too much heat will change the color of the stone, usually washing out the color; heat will also make an oiled/waxed gem "sweat" a little.
Sometimes "reconstructed" turquoise is seen on the market; this is small bits of natural turquoise glued together. Synthetic turquoise has also been available for about 25 years. On extremely rare occasions, clear turquoise crystals are found in nature -- these are very expensive.