Feature Gems & Gemology, Winter 1991, Vol. 27, No. 4

Sunstone Labradorite from the Ponderosa Mine, Oregon

The Ponderosa mine is now producing commercial quantities of the gem-quality labradorite feldspar known as sunstone. This gem material exhibits unique optical and physical properties that include a wide range of saturated body colors, aventurescence, and strong pleochroism. The feldspar is An70 labradorite, a high calcic plagioclase; the aventurescence-causing inclusions have been identified as pure native copper. Microprobe analyses conducted on colorless, red, and green samples did not reveal any obvious causative mechanism for the exhibited colors, although ongoing research is investigating intervalence charge transfer, possibly involving colloidal copper. Gemological identification involves the use of R.I., S.G., and microscopy. Locality determination (Oregon) is based on the copper inclusions and a distinct pleochroism.