Sunstone – Oregon, USA
The 1987 Oregon Legislature designated Oregon sunstone as the official state gemstone. Uncommon in its composition, clarity and colours, it is a large, brightly coloured transparent plagioclase feldspar gem that occurs in Lake and Harney Counties at the Plush area Dust Devil mine and Ponderosa mines respectively. The largest pieces are recovered from the soil and the underlying, primarily weathered, basaltic lava flows.
Compositionally, the Oregon sunstones are mixed crystals in the plagioclase solid solution series with a range of Na:Ca ratios. The most abundant type is labradorite (An70) but bytownite, andesine and oligoclase have all been reported.
Oregon sunstone contains native copper inclusions that produce a variety of colours. Additionally these millions of tiny colloidal copper particles, when sufficiently large (22 nm), reflect light with varying intensities resulting in a golden red play of colour known as schiller (a shining surface); akin to aventurescence (a spangling or glittering
caused by light reflecting of small or minute particles respectively).
Commonly it is straw-yellow coloured, but it can also be pink, peach, red, salmon red-orange, red-green and blue-green. It also can be bicoloured and tricoloured in combinations of yellow, green and red arranged concentrically
in the rough. Asmall percentage is di- and tri-chroic. Copper content is variable and affects the colour: pale yellow stones have a copper content as low as 20 ppm, greenstones about 100 ppm and red stones up to 200 ppm. Colloidal size is also a factor affecting colour and intervalency charge transfer (Cu0–Cu+) and Cu0 pairs may also play a part. The reported colour change may be due to the effect large copper colloids has when the stones are viewed in incident light.
Ponderosa sunstone can be found as incredibly saturated reds, with a lot of copper schiller. Dust Devil sunstones appear more in peach and green colours, with less schiller, and in a wider range of colours than
Ponderosa material. Both localities produce gems with strong red-green
Plush material RIaverage: 1.564; 1.569; 1.576. Biaxial ve/ve. SG:
2.725 average over a range of 2.712–2.736. Ponderosa mine RIaverage
1.564; 1.568; 1.573. SG 2.67–2.72. Biaxial ve. SG 2.70. Oregon oligoclase
Ab86An13Or1 – RI 1.535–1.536, 1.538–1.539, 1.545–1.547.