Owyhee Jasper
Owyhee Jasper
Patterned chalcedony from the high desert of the Oregon-Idaho border
Owyhee jasper is the trade name for a group of patterned, opaque-to-translucent chalcedony materials recovered from the Owyhee region of the Oregon-Idaho-Nevada borderlands in the western United States. The name covers a number of distinct deposits — Bruneau, Willow Creek, Wild Horse, and Biggs among them — that share a broadly similar geological setting in altered volcanic rock and a comparable visual character of bold landscape-like patterning. Owyhee jasper is one of the principal North American sources of decorative jasper material and is a regular fixture in the cabochon and lapidary trade.
Geological setting
The Owyhee deposits sit in the volcanic terrane of the Owyhee Plateau and the southwestern margin of the Snake River Plain. The host rocks are Miocene rhyolitic tuffs and silicic volcanic flows, which were variably altered by hydrothermal circulation in the late stages of the volcanic episode. Silica liberated by the alteration reprecipitated as chalcedony in fissures, vughs, and replacement zones, with the patterns and colours of the resulting material reflecting the chemistry of the depositing fluids and the iron and manganese content of the host rock.
The Owyhee material is technically chalcedony — microcrystalline quartz — rather than true jasper in the older mineralogical sense, but trade usage applies jasper to the opaque, patterned chalcedonies of the region and the convention is now well established. Hardness is 6.5 to 7 on the Mohs scale, specific gravity around 2.6, and refractive index approximately 1.54. The material takes a high polish.
The principal deposits
Bruneau jasper, from the Bruneau River area in southwestern Idaho, is the most widely known of the Owyhee jaspers, characterised by rich brown, beige, and red banding in flowing landscape patterns. Willow Creek jasper, also from southwestern Idaho, runs to softer cream, peach, and pastel pinks with subtle banding. Wild Horse jasper from the Owyhee Mountains shows bolder colour combinations with stronger reds and greens. Biggs jasper from north-central Oregon, often included under the broader Owyhee label, is a finely banded picture jasper with brown, gold, and blue-grey tones in the so-called landscape patterns that have made the material a long-standing favourite of cabochon cutters.
Cutting and use
Owyhee jasper is cut almost exclusively as cabochon, both as freeform shapes following the natural pattern of the rough and as standard millimetre calibres for production jewellery. The cutter selects the saw orientation to display the most desirable pattern across the cabochon face and orients the dome to centre the most striking element of the design. The material is also used in beads, ornamental objects, and inlay work. Polishing is straightforward; the material accepts a high lustre with standard cerium oxide or alumina compound.
The market
Owyhee jasper trades principally through the Tucson Gem and Mineral Show, the Quartzsite shows, and the regional rock-and-mineral circuit in the western United States, with international distribution through specialist dealers. Pricing is moderate; commercial-grade material trades in the low single-digit dollar-per-carat range, with the most striking landscape and picture cabochons reaching higher levels for collector pieces. The material has been continuously available since the 1960s, although individual deposits run hot and cold depending on production from the small-scale workings that supply the market.
Care
Owyhee jasper is durable and chemically stable in normal wear. Cleansing should be by warm soapy water and soft brush; ultrasonic and steam cleaning are generally safe but should be used with caution on cabochons with significant matrix or fracture content. The material is not heat-sensitive in the normal range, although thermal shock should be avoided.