| Sunstone | |
|---|---|
 | |
| General | |
| Category | Tectosilicateminerals,feldspargroup,microcline oroligoclasevariety |
| Formula | (Ca,Na)((Al,Si)2Si2O8) |
| Crystal system | Triclinic |
| Identification | |
| Color | Colorless, orange, yellow, red, green, blue, brown, and copper shiller |
| Crystal habit | Euhedral crystals,granular |
| Twinning | Lamellar |
| Cleavage | 001 |
| Mohs scale hardness | 6.0–6.5 |
| Diaphaneity | Transparent to translucent and opaque |
| Specific gravity | 2.64–2.66 |
| Refractive index | 1.525–1.58 |
Sunstone is amicrocline oroligoclasefeldspar, which when viewed from certain directions exhibits aspangled appearance. It has been found inSouthern Norway,Sweden, various localities in theUnited States, and on some beaches along the midcoast ofSouth Australia.
The optical effect is due to reflections from inclusions of redcopper, hematite, or goethite, in the form of minute scales, which arehexagonal,rhombic, or irregular in shape, and are disposed parallel to the principalcleavage-plane. These inclusions give the stone an appearance something like that ofaventurine, hence sunstone is known also as "aventurine-feldspar". The optical effect is called schiller and the color of 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 isoligoclase, though the effect is sometimes seen inorthoclase: Hence two kinds of sunstone are distinguished as "oligoclase sunstone" and "orthoclase sunstone".
Sunstone was not popular until recently.[when?] Previously the best-known locality beingTvedestrand, nearArendal, in southNorway, where masses of the sunstone occur embedded in a vein ofquartz running throughgneiss.
Other locations include nearLake Baikal inSiberia, and severalUnited States localities – notably atMiddletown Township, Delaware County, Pennsylvania;Plush, Oregon; andStatesville, North Carolina.
The "orthoclase sunstone" variant has been found nearCrown Point and at several other localities inNew York, as also atGlen Riddle inDelaware County, Pennsylvania, and atAmelia Courthouse,Amelia County, Virginia.
Sunstone is also found inPleistocenebasalt flows at Sunstone Knoll inMillard County, Utah.[1]
A variety known as "Oregon sunstone" is found inHarney County, Oregon and in easternLake County north ofPlush. Oregon Sunstone contains elementalcopper.[2] Oregon Sunstone is unique in that crystals can be quite large. 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.[3]
On 4 August 1987, theOregon State Legislature designated Oregon Sunstone as itsstate gemstone by joint resolution.[4]
In the early 2000s, a new variety of red or green gemstone resembling sunstone and called "andesine" appeared in the gem market. After much controversy and debate, most of these gemstones, allegedly sourced from China, were subsequently discovered to have been artificially colored by a copper diffusion process.[5] ATibetan source of bona fide (untreated) red andesine, however, was eventually verified by a number of independent groups of well-respected gemologists.[6][7][8][9][10]
This article incorporates text from a publication now in thepublic domain: Chisholm, Hugh, ed. (1911). "Sunstone".Encyclopædia Britannica. Vol. 26 (11th ed.). Cambridge University Press. p. 110.
