Oregon Sunstone is a distinct type of feldspar gemstone known for its exclusive source location. This transparent to translucent mineral, a member of the plagioclase feldspar group, is prized by gem enthusiasts and jewelers for its clarity and remarkable internal glow. It is noted for a unique optical phenomenon that sets it apart from other sunstones. The gem’s origins are deeply intertwined with the volcanic history of the American West.
Geological Identity and Composition
Oregon Sunstone is classified as a calcic plagioclase feldspar, specifically within the labradorite family. Its core chemical structure is a sodium-calcium aluminum silicate, but its unique properties stem from native copper particles suspended within the crystal lattice. As the host rock cooled, trace amounts of elemental copper exsolved (separated from the silicate melt). This process formed countless microscopic, flat copper platelets that became trapped within the growing crystal. These copper platelets, often measuring between 1 and 100 micrometers in diameter, are the foundation for the gemstone’s distinctive appearance, differentiating it from other sunstones which typically contain hematite or goethite.
The Source and Official Status
The only known commercial source of this copper-bearing sunstone variety is located in the high desert region of Eastern Oregon. Deposits are concentrated in Harney and Lake counties, notably around the town of Plush. The gemstone’s formation is directly linked to ancient volcanic activity, specifically basaltic lava flows that occurred 15 to 17 million years ago. The crystals formed as phenocrysts, large crystals embedded in the solidified lava rock of the Columbia River Basalt Group. This scarcity led the state legislature to officially designate it as the Oregon State Gemstone in 1987, recognizing its importance to the state’s natural heritage.
Unique Optical Properties (Aventurescence)
The most captivating feature of Oregon Sunstone is its unique optical effect, known as aventurescence or schiller. This metallic, spangled flash of light shimmers from within the stone as it is moved. The phenomenon is caused by light reflecting off the numerous, minute copper platelets uniformly aligned within the feldspar structure. The intensity and color of the schiller are directly related to the size and density of these copper inclusions. Smaller, dispersed platelets typically produce a subtle, reddish-gold sheen, while larger, concentrated inclusions create a dramatic sparkle. The copper content also contributes significantly to the stone’s body color, which ranges from clear or champagne to soft yellows, pinks, and peaches. Rarer specimens showcase deep, saturated hues of red and green, and the most exceptional stones are bi-color or tri-color, displaying distinct zones of color within a single gem.
Uses in Jewelry and Value Factors
Oregon Sunstone is widely used in jewelry, valued for its beauty and moderate durability. The stone registers between 6.5 and 7.2 on the Mohs scale of hardness, making it suitable for various applications, though it requires more care than harder gems like sapphire. Its unique properties are highlighted through custom cuts, including brilliant faceting for transparent stones and cabochon cutting for those with dense schiller. The market value is determined by factors beyond the standard four Cs. While carat weight and cutting quality are important, the intensity of the schiller and the rarity of the body color are paramount. Deep red and forest green stones command the highest prices due to their scarcity. Unlike many other gemstones, the presence of the copper inclusions that cause the aventurescence is considered a positive clarity factor, adding to the stone’s distinctiveness.