Rare Opals:

 

Opal is rare.  Opal, spinel and pearls are the only "gems" that cannot be manufactured, treated or mimicked to such a quality as to be indistinguishable from natures wonder.

There are only a few locations in the world where it is found in commercial quantity and of sufficient grade to warrant the effort.  Australia produces 90% of the world's opal from the following regions: