Oregon Sunstone — Copper-Bearing Plagioclase from the Lake County Deposits
Oregon Sunstone — Copper-Bearing Plagioclase from the Lake County Deposits
A copper-coloured feldspar with aventurescent schiller, the only significant red-feldspar source in the world
Oregon sunstone is a copper-bearing plagioclase feldspar from the southeastern Oregon high desert, occupying a unique position in the gem trade as the only significant source of red transparent feldspar with copper aventurescence. The material is produced principally in Lake County, near the small town of Plush, and is the official state gemstone of Oregon. Its colour palette runs from colourless and pale yellow through pink, salmon, and saturated red, with green stones appearing in unusual chemistries; the schiller — the metallic flash from oriented copper platelets — is the optical signature that distinguishes Oregon sunstone from other aventurescent feldspars worldwide.
Composition and colour
Oregon sunstone is a labradorite-andesine plagioclase, typically falling in the An20-An30 compositional range, with copper present both as a colouring trace element in solid solution and as oriented platelets that produce aventurescence. The two roles of copper are usually correlated: stones with stronger schiller tend also to be more strongly coloured, although colourless transparent stones with strong aventurescence and saturated red stones with weak schiller both occur. Colour zoning is common, and bicolour and polychrome material with sharp boundaries between green and red, or red and yellow, is a characteristic of the deposit and a value-add for collectors.
Refractive index runs approximately 1.532 to 1.542, specific gravity is in the range of 2.62 to 2.65, and hardness is 6 to 6.5 on the Mohs scale. The species is triclinic and shows weak to moderate pleochroism in the more strongly coloured material. Copper concentrations in the most saturated red stones are reported in the literature at hundreds of parts per million, an order of magnitude higher than in pale and colourless material.
The Plush deposits
Sunstone occurs as phenocrysts in basalt flows of the Steens Mountain volcanic series, with the gem-quality material recovered both from in-place mining of host rock and from placer concentrations weathered out into the desert soils. The principal commercial mines include the Ponderosa Mine in Harney County and the Sunstone Butte and Dust Devil mines in Lake County. Sunstone Knoll, on Bureau of Land Management land, is open to public collecting under permit and is responsible for a steady stream of small-stone material into the lapidary trade.
Production is small by international standards. A reliable estimate of total annual gem-grade output across all Oregon mines is in the low hundreds of kilogrammes of cuttable rough; saturated red material with strong schiller is a small fraction of that figure.
Cutting and grading
Oregon sunstone is faceted in standard brilliant and step-cut styles, with cushion and oval modified brilliants the most common in commercial production. Cabochon material is reserved for the strongest-schiller, most aventurescent stones and for translucent rough that does not facet cleanly. Pricing is driven by colour saturation, schiller intensity, transparency, and size, with a strong premium on stones above 5 carats and a further premium above 10 carats. Bicolour stones with clean colour boundaries trade at a separate, often higher, tier than monochrome material of the same weight.
Treatment of Oregon sunstone is essentially absent. The material is sold untreated under disclosure standards consistent with American Gem Trade Association practice, and laboratory verification of natural colour and natural copper distribution is straightforward where it is required.
In the trade
Oregon sunstone occupies a specific niche between the major coloured-stone categories. It is the material of choice for jewellery brands building origin-led narratives around domestic American supply chains, for designers who value untreated material with a documented chain of custody from a small number of identifiable mines, and for collectors who recognise its uniqueness as the only significant copper-bearing red transparent feldspar in the world. Buyers should distinguish Oregon sunstone from Indian aventurescent oligoclase, Tanzanian sunstone, and Andean andesine, all of which trade at substantially different price points and have different gemmological histories.
See also sunstone, Plush, Ponderosa, oligoclase.