I do like unusual and rare gemstones and this one certainly fits the bill (as well as being beautiful!). 

Tiffany stone, while not associated with Tiffany and Company, is sometimes sold as Opalized Fluorite or Bertrandite (although it actually contains very little of this mineral).

Tiffany stone is a fluorite in various soft tones of purple which may contain small impurities of inter-grown bertrandite, calcite, dolomite, chalcedony, opal and/or quartz. The stone is brittle and depending on its exact composition it may have a Mohs’ hardness of 4, equivalent to that of fluorite (a higher quartz or chalcedony content may increase the hardness to 5). This makes it less suitable for rings or bracelets but fine for pendants, brooches and earrings.

Found only in Utah, USA, the patterns seen in this opaque stone are all individual - rare, beautiful, individual and purple, what's not to love about this stone!