As the links Jamey posted show, I have a fetish about accurate information about varieties of turquoise as well as the imitations of turquoise. I'll add a few comments here and try to collect all my pictures and post them in a turquoise specific thread.

For several years Fire Mountain Gems mislabeled blue dyed magnesite as "chalk Turquoise" describing it as a real turquoise without the copper. 10 months ago their ask the expert column posted "Chalk turquoise is a form of natural turquoise that has a white chalk-like consistency. It has the same chemical composition as turquoise, only without the copper (it's the copper that causes the blue turquoise color). Chalk turquoise is also a bit softer than regular turquoise. It is dyed pleasing colors and stabilized with resin to produce beads that are hard enough to use in jewelry. Chalk turquoise is considered less valuable than regular turquoise because it does not contain the minerals that create the rich blue and green colors."

Turquoise is a hydrated phosphate of copper, aluminum and often iron. It MUST have copper or it is a different mineral. FMG's description is not accurate.

A google search for "Chalk turquoise is a form of natural turquoise" shows the many sites that quote this inaccurate description.

Dyed magnesite is an imitation turquoise.
I agree with other posters that most "Tibetan" turquoise originated in China. Chinese turquoise comes in many different qualities and colors - some is used natural but most is now stabilized with resin.

The picture below from my Flickr photo pages shows genuine Mexican turquoise in natural and stabilized form. The large piece is natural Mexican turquoise (Competos mine, Sonora) showing the typical pyrite inclusions. This is soft , very light blue turquoise often called "chalk turquoise" that must be stabilized before it is useful in jewelry. The blue nuggets around the rough have been stabilized in plastic resin. In a few cases the resin did not penetrate to the center of the nugget causing the paler center in the cracked open nuggets.