The Sunstone

Also known as heliolite,

is a transparent feldspar with colors ranging from water clear through pale yellow, soft pink, and blood red to (extremely rare) deep blue and green. The color appears to vary systematically with small amounts of copper and may depend on both the amount and the size of individual copper particles present in the stone.

Pale yellow stones have a copper content as low as 20 parts per million (ppm) (0.002 percent), green stones contain about 100 ppm per million (0.01 percent), and red stones have up to 200 ppm (0.02 percent) copper. Some of the deeper colored stones have bands of varying color, and a few stones are dichroic, that is, they show two different colors when viewed from different directions.

Many stones appear to be perfectly transparent at first, but when they are viewed in just the right direction, a pink to red metallic shimmer flashes from within the stone. This effect is called "schiller" or "aventurescence" and is caused by light reflecting from minute parallel metallic platelets suspended in the sunstone. When viewed along their edges, the platelets are invisible to the naked eye; when viewed, however, perpendicular to their surfaces, they reflect light simultaneously from each platelet, creating a mirror effect. Earlier studies of the Lake County feldspar suggested that the platelets were hematite (iron oxide), but the most recent research concludes that they are flat crystals of copper metal.

The terms "sunstone" and "heliolite" (from Greek helios, meaning sun, and lithos, meaning "stone") have been used for at least two centuries for feldspars exhibiting schiller. The Lake County occurrence was first reported in 1908, and the presence of the schiller effect was the original reason for naming the stones sunstones. For decades, however, the term "sunstone" has been used for these Oregon gem feldspars both with and without schiller.

Oregon sunstones are a calcium-rich variety of plagioclase feldspar named labradorite, a common mineral in basaltic lava flows. All three known sunstone occurrences are in small basalt flows that superficially resemble basalt flows elsewhere in the state that contain large feldspar phenocrysts or megacrysts. However, feldspars in those flows are typically cloudy to opaque and relatively small compared to those in the sunstone flows, which are clear, glassy, and can be up to 2 or 3 in. in one dimension.

No detailed information has been collected on the geology, petrography, or chemistry of the known sunstone flows, so no meaningful comparisons can be made between them or with other flows in the area. The sunstone flows appear to be small; the Lake County occurrence covers about 7 sq. mi., and the two Harney County occurrences are probably less than 1 sq. mi. each. Considering the regional geology and the wide separation between the flows, it is probable that there are more sunstone occurrences in the area.

Sunstones are mined from the soil and partially decomposed rock formed by weathering of the lava flows. The surface debris is dug with pick and shovel and sieved through a quarter-inch screen, and the sunstones are separated from rock fragments by hand. In some local areas, the lava flows are weathered to a depth of several feet, and good stones have been recovered from pits dug into these zones. Hard-rock mining techniques have been used on unweathered parts of the flows, but the sunstones are often shattered along with the lava, and recovery of large unbroken stones is difficult.

Except for part of the Lake County occurrence, all three producing areas are held by mining claims and are not available for collecting without permission of the claim owners. About 2 sq. mi. of the Lake County flow have been withdrawn from mineral entry and established by the U.S. Bureau of Land Management (13LM) as a free public collecting area. This sunstone area is located off the northeast flank of the Rabbit Hills about 25 mi. north of Plush and 80 mi. northeast of Lakeview. Maps, directions, and information on road conditions are available from the BLM District Office in Lakeview.

Varieties of feldspars used as gemstones are valued for their colors or optical effects. Being typically translucent to opaque, they are normally cut in rounded forms or cabochons. Transparent gem feldspars, particularly calcium-rich varieties, that can be cut as faceted stones are rarer. Occurrences of transparent labradorite have been reported from Arizona, California, New Mexico, and Utah, but few gems have been produced from those areas. Oregon sunstones are uncommon in their composition, clarity, and range of colors, and they occur in sufficient abundance to permit sustained production of faceted gems.

Editors Note: This article Oregon Sunstones reprinted from an February, 1987 Oregon Geology article by Ron Geitgey, DOGAMI..