Most of the turquoise on the market has been treated in some way to deal with two problems intrinsic with this gemstone. First, turquoise is soft, sometimes even brittle, and thus susceptible to facture. Second, turquoise is porous.
Another problem is that quality turquoise is getting harder to find. Many mines have been depleted and are now closed. The turquoise coming out of other mines is of lower quality than was once produced. As a result, methods have been developed to create turquoise with the appearance of quality by dying pale shades and by creating stones out of "chalk turquoise" (called reconstituted turquoise).
It should be noted that turquoise has been treated for thousands of years, usually with waxes and oils. Modern techniques represent "advances" on old solutions. Furthermore, many of the various types of gemstones sold these days have been treated in some way to harden them and create clearer crystals; heat treating is especially common.