I Am Rare, Stunning and Contain a Gemological Mystery in My Core

I AM ONE OF the world’s rarest gemstones and one of gemology’s greatest mysteries — science has been unable to explain my color, nor has anyone been able to artificially reproduce my crystal structure. First found in California’s southern Mount Diablo range in 1906, I am today that state’s official gemstone. Shreve & Co., the famed San Francisco store, led the drive to market me as a jewelry stone but despite my stunning brilliance — my dispersion is only slightly less than that of a diamond — I have never caught on with consumers (I am a little soft and usually very small in gem quality samples). Nevertheless, I remain highly prized among collectors.

Who am I?