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Considerable work has recently been done on the geology of eastern Oregon, principally under the aegis of the High Lava Plains Project, supported by the Carnegie Institute of Washington’s Department of Terrestrial Magnetism (ciw.edu/research/HLP).
This project sought to explain Oregon’s recent (20 Ma) volcanic history through the work of specialists in petrology, geochemistry, geochronology, and geophysics.
This superb 2.85 ct sunstone from Sunstone Butte displays the gem’s most valued attributes: a blend of green and red bodycolor, with reflective spangles of native copper glittering in the interior. No discussion of this topic would be complete without mention of the controversy surrounding treated copper-bearing feldspar; Rossman (2011) provides a chronology.
In the early 2000s, Asian treaters perfected a method of diffusing copper into pale feldspar, flooding the market with low-priced, attractive red and green gems—which destabilized the market for Oregon sunstone.
Past stratovolcanoes of the High Cascades and across Oregon’s remote High Lava Plains lie three sunstone mines (figure 3).
All three produce labradorite feldspar ranging from near-colorless to pale yellow to red and green, including bicolor specimens.
The sunstone often contains tiny reflective platelets of native copper, referred to as “schiller.” From Portland, we drove five hours southeast to Ponderosa, near the town of Burns.
We spent three days there before driving three hours southwest to spend the next two days at the Dust Devil and Sunstone Butte mines.
Geologists debate whether the volcanic events were caused by melting of subducted oceanic crust or by a hot plume in the earth’s mantle.In a future visit, we aim to collect representative samples from plagioclase-rich flows at Abert Rim and Steens Mountain and compare them with material from each of the three mines to see if we can confirm their relationships.