Three Occurrences of Oregon Sunstone

ABSTRACT

Over five days in late July 2013, we visited three important sources of gem-quality sunstone in eastern Oregon: the Ponderosa mine in Harney County, and the Dust Devil and Sunstone Butte mines, both of which are about 120 miles further south, in Lake County.

INTRODUCTION

Oregon sunstone, the official state gemstone since 1987, is natural copper-bearing labradorite feldspar. Some examples exhibit red to green dichroism, and fine specimens larger than 5 ct can sell for more than $1,000 per carat.

Feldspars are silicate minerals that contain variable amounts of sodium (Na), potassium (K), and calcium (Ca). Labradorite belongs to the plagioclase feldspar series, which forms a solid solution between anorthite (CaAl₂Si₂O₈) and albite (NaAlSi₃O₈). Oregon sunstone, which contains substantially more calcium than sodium, has a composition of about 70% anorthite and 30% albite, or An⁷⁰/Ab³⁰. For more on feldspar classification and nomenclature, see box B in Rossman (2011).

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. Promoted as “andesine” and purportedly from mines in Congo or Tibet, this material was subsequently found to be treated, but the resulting furor hindered the public’s trust in natural copper-bearing feldspar. Despite this setback, Oregon sunstone miners have strived to rebuild the market, and there are signs of renewed consumer interest in their one-of-kind gem.