Semi-Precious Gemstones: Sunstone
Sunstone (Adventurine Feldspar) used in Jewelry
Source: Australia, Canada, India, Norway, Russia, Tanzania, USA (Oregon Sunstone)
Sunstone (aka: adventurine or aventurine feldspar) is a transparent to translucent tectosilicate which is a member of the feldspar group (orthoclase feldspar). This colorless, brown, orange, or red gemstone is found as a constituent rock-forming component in feldspar-rich granitic and syenitic pegmatites. The Norse Vikings revered a mineral which they called "sunstone" that was actually a transparent variety of calcite called "Iceland spar" that is totally unrelated to the aventurine feldspar variety of sunstone.
Sunstone, moonstone, and labradorite are all comprised of feldspars, and red sunstone is sometimes referred to as "red labradorite." Sunstone can be included with tiny flecks called "snowflakes," consisting of copper or hematite. Sunstone is a relatively rare gem that occurs in few locations which include southern Norway (Tvedestrand), Russia (Lake Baikal, Siberia), and the United States (eastern Oregon).
Oregon Sunstone
Oregon Sunstone (aka: red labradorite, yellow labradorite, andesine) is a sodium-calcium-aluminum-silicate variety of plagioclase feldspar that is found within basalt lava flows. The coloring of Oregon Sunstone is caused by clouds of microscopic copper platelet inclusions (snowflake schiller). Oregon Sunstone is a unique mineral that is found in only three locations of central Oregon's high desert: the Ponderosa Sunstones Mine in Harney County, the Onion Mountain Quarry in Josephine County, and the Spectrum Sunstone Mine and Dust Devil Mine in Lake County, Oregon.
Oregon sunstone rough |
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Opaque sunstone with snowflakes (photo: Dan Dennis) |
The addition of copper provides Oregon Sunstone with a vivid burgundy-red, reddish-brown, mauve, or orange-red color which is sometimes confused with red andesine [2]. Oregon Sunstone can be faceted or cut en cabochon. The Spectrum Sunstone Mine in Plush, Oregon was once owned by Tiffany & Co. of New York, and is Oregon's first commercially operated sunstone mine. Oregon Sunstone was designated as Oregon's official state gemstone on August 4, 1987.
Sunstone's Aventurescence
Sunstone's characteristic glistening or shimmering effect is caused by an optical phenomenon that is referred to as the "schiller effect," "aventurescence," or "aventurization." Aventurescence is caused by thin, uniformly oriented, platy mineral inclusions (adularia) and alternating layers of feldspar, orthoclase (Greek for "straight fracture"), and plagioclase feldspar (plagioclase is Greek for "oblique fracture") which is a variety of albite, sheet mica, and orthoclase. These layers all interact to scatter reflected light when incident light rays refract off of lamellas (lamellar intergrowths) inside the stone.
Oregon Sunstone Crystallography, Chemistry, Physical Properties
Oregon Sunstone Optical Properties
Crystal twinning is a common occurrence in most plagioclases. Basal cleavage forms right angled prisms with good cleavage in one of the two directions, and perfect cleavage in the other.
Sunstone is cut en cabochon when it is opaque, but transparent gem varieties can be faceted as well. The most desirable effects of aventurescence are achieved when the sunstone is cut on its optimal crystal axis. Oregon sunstone's color usually occurs in a centralized zone, surrounded by a clear or light-colored rind. Sunstone has a critical angle of 41 to 45 degrees. [6].
The above YouTube video shows the cutting/faceting of a 300+ carat flawless red Oregon Sunstone (the "Spectrum Sunset") that was discovered at the Spectrum Mine in 2006.
Bibliography and Reference on Oregon Sunstone
1. Paul R. Shaffer, Rocks, Gems and Minerals  . Martin's Press
2. Exotic Gemstones LLC., About Oregon Sunstone . www.oregon-sunstone.com
3. Dust Devil Mining Co., Oregon Sunstone from the Dust Devil Mine . www.dustdevilmining.com
4. Ponderosa Mine, Sunstone from the Ponderosa Mine . www.oregonsunstone.com
5. High Desert Gems & Minerals, Spectrum Sunstone Mine . www.highdesertgemsandminerals.com
6. Dust Devil Mining Co., Cutting Oregon Sunstone . www.dustdevilmining.com
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