Turquoise is an unstable gemstone in its natural state, because chemicals and body oils easily damage it. Durable stones won't scratch or break when worn. Soft stones, like opal, must be protected if they are to last. The four main methods of enhancement are heat treatment, irradiation, chemical treatment, and assembly. While some enhancements are done with an intent to deceive, many gemstones would not be usable in jewelry if they were not treated in some way.
Reputable suppliers disclose known enhancements at the time of purchase.
The results of heat treatment and irradiation often mimic what Mother Nature would have achieved had the stones been left in the ground a few more centuries. It is usually impossible to detect which gemstones have been enhanced by either of these methods. Chemical enhancements are generally used to change the surface characteristics of gemstones, and assembly is often used to protect fragile stones. Heat treatment is commonly used to improve color. Color changes resulting from heating are permanent in most gemstones.