Tiffany Stone is a form of Opal and Bertrandite, and is a super rare lapidary material from Utah. It is generally a purple, white, and black opal, but it can also more rarely have blue and orange! To my knowledge, it is now a private government owned mine, with Tiffany Opal being mined and crushed for the precious Bertrandite beryllium metal within! Beryllium is often used in aerospace and rocket manufacturing, due to it being a lightweight metal with excellent thermal stability and conductivity.
Tiffany Stone makes some excellent cabochons, and even prior to the mine being government operated, it was a very high demand stone! With this material being super rare, you never want to miss out on a chance to get some!
Measurements (inches - longest - approx): 4.6 x 2.7 x .28
The last photo has been taken dry so that you may see any defects. The other photos have been taken wet and in natural lighting to simulate a polish.