|Turquoise is another stone that dates back at least 5000 years to ancient Egypt and Persia. The name comes from the word for Turkish although it was not mined in Turkey but traded there along caravan routes since ancient times. It means "sky blue".
It is rarely perfectly blue but usually has other colors and patterns running through the stone. Often mixed minerals that are black, brown, greenish, tan, or dark gray. Several varieties of turquoise contain complete secondary minerals like pyrite or other metal sulfides. The composition of the turquoise can often be used to place it to a particular mine in the southwestern United States.
It does not form crystals and appears only in masses and seams, thus there are no transparent faceted stone, and it is mainly cut into cabochons. Cabochons have been "oiled" or "waxed" for a thousand years and this has been a generally accepted treatment. The oiling gives the stone a wetted look and intensifies the color. This is an external treatment and not to be confused with the "stabilized variety" described below.
When heated it loses water easily and may turn various shades of green. Its structure is very inconsistent and some of the green may be due to the presence of iron. It can be absorbent and porous, and can be attacked by strong acid. Low grade turquoise is often chalky in nature and very porous. This feature has been used to make a "stabilized" turquoise.
Stabilized turquoise is pressure treated chalky turquoise in the presence of polymers. the polymers are absorbed at higher pressure and create a "plastic" containing chalk with better physical properties, but of less value than the natural material. One test for stabilization it take a hot pin and place it against the bottom of a cabochon, stabilized turquoise will often smoke and smell like burned plastic.
Natural turquoise can run from sky blue, to dark blue and virtually all shades of green. There is sometimes a material miss-named "red turquoise", but it is actually the mineral alunite and is not related to turquoise in any way.