|sodium calcium aluminum silicate (Ca,Na)((Al,Si)2Si2O8)|
|Color||clear, yellow, red, green, blue, and copper shiller|
|Crystal habit||Euhedral Crystals, Granular|
|Diaphaneity||Transparent to Translucent|
|Optical properties||Double Refractive:|
Sunstone is a plagioclase feldspar, which when viewed from certain directions exhibits a brilliant spangled appearance; this has led to its use as a gemstone. It has been found in Southern Norway, and in some United States localities. It is the official gemstone of Oregon.
The optical effect appears to be due to reflections from inclusions of red copper, in the form of minute scales, which are hexagonal, rhombic, or irregular in shape, and are disposed parallel to the principal cleavage-plane. These inclusions give the stone an appearance something like that of aventurine, hence sunstone is known also as "aventurine-feldspar." The optical effect called shiller and the color in Oregon Sunstone is due to copper. The middle part of this crystal sparkles, and usually the color is darkest in the middle and becomes lighter toward the outer edges.
The feldspar which usually displays the aventurine appearance is oligoclase, though the effect is sometimes seen in orthoclase: hence two kinds of sunstone are distinguished as "oligoclase sunstone" and "orthoclase sunstone."
Sunstone was not common until recently. Previously the best-known locality being Tvedestrand, near Arendal, in south Norway, where masses of the sunstone occur embedded in a vein of quartz running through gneiss.
Other locations include near Lake Baikal in Siberia, and several United States localities—notably at Middletown Township, Delaware County, Pennsylvania; Plush, Oregon; and Statesville, North Carolina.
The "orthoclase sunstone" variant has been found near Crown Point and at several other localities in New York, as also at Glen Riddle in Delaware County, Pennsylvania, and at Amelia Courthouse, Amelia County, Virginia.
In the early 2000s, a new variety of red or green gemstone resembling sunstone and known as 'Andesine' appeared in the gem market. After some controversy, these gemstones were subsequently discovered to have been artificially-coloured.
A variety known as Oregon sunstone is found in Harney County, Oregon and in eastern Lake County north of Plush. Oregon sunstone contains inclusions of copper crystals. Oregon sunstones can be up to three inches wide. The copper leads to variant color within some stones, where turning one stone will result in manifold hues: the more copper within the stone, the darker the complexion.
- Sunstones at Sunstone Knoll, Millard County. Utah Geological Survey, accessed September 14, 2007.
- "Rock Hounding: Oregon Sunstone--Official State Gemstone". Nature of the Northwest (Oregon Department of Geology and Mineral Industries). Retrieved 7 March 2013.
- Chapter 186 – State Emblems; State Boundary 2005 Oregon Revised Statutes
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