Turquoise is hydrated aluminum copper phosphate that can contain iron as well. Nearly all turquoise on the market today is stabilized to retain its shape, as it is a very soft stone. There are very few types of turquoise found in the world that remain stable on there own.
When turquoise is stabilized it is injected with vaporized quartz, which is undetectable by normal testing methods. The process, however, does not work on chalk turquoise. Chalk turquoise is light colored turquoise that must be stabilized using other methods. Although, lately when people say chalk turquoise they mean dyed magnesite.
Stabilized turquoise is often confused with reconstituted turquoise which is man made out of stone dust, small pieces of stone, dye and a plastic binder.