Sunstone is a microcline or oligoclase

feldspar Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocl ...

, which when viewed from certain directions exhibits a spangled appearance. It has been found in

Southern Norway Southern Norway (; lit. "The Southland") is the geographical List of regions of Norway, region (''landsdel'') along the Skagerrak coast of southern Norway. The region is an informal description since it does not have any governmental function ...

Sweden Sweden, formally the Kingdom of Sweden, is a Nordic countries, Nordic country located on the Scandinavian Peninsula in Northern Europe. It borders Norway to the west and north, and Finland to the east. At , Sweden is the largest Nordic count ...

, various United States localities and on some beaches along the midcoast of South Australia.

Properties

Physical properties

The optical effect is due to reflections from inclusions of red

copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...

, hematite, or goethite, 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 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 is oligoclase, though the effect is sometimes seen in

orthoclase Orthoclase, or orthoclase feldspar ( endmember formula K Al Si3 O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture", because its two cleavage planes are at right angles ...

: hence two kinds of sunstone are distinguished as "oligoclase sunstone" and "orthoclase sunstone".

Distribution

Sunstone was not popular until recently. Previously the best-known locality being Tvedestrand, near Arendal, in south

Norway Norway, officially the Kingdom of Norway, is a Nordic countries, Nordic country located on the Scandinavian Peninsula in Northern Europe. The remote Arctic island of Jan Mayen and the archipelago of Svalbard also form part of the Kingdom of ...

, where masses of the sunstone occur embedded in a vein of

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

running through gneiss. Other locations include near

Lake Baikal Lake Baikal is a rift lake and the deepest lake in the world. It is situated in southern Siberia, Russia between the Federal subjects of Russia, federal subjects of Irkutsk Oblast, Irkutsk Oblasts of Russia, Oblast to the northwest and the Repu ...

Siberia Siberia ( ; , ) is an extensive geographical region comprising all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has formed a part of the sovereign territory of Russia and its predecessor states ...

, and several United States localities—notably at Middletown Township, Delaware County, Pennsylvania; Plush, Oregon; and Statesville, North Carolina. A small polished sunstone

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 Delaware County, colloquially referred to as Delco, is a County (United States), county in the Commonwealth (U.S. state), Commonwealth of Pennsylvania. With a population of 576,830 as of the 2020 census, it is the List of counties in Pennsylv ...

, and at Amelia Courthouse, Amelia County, Virginia. Sunstone is also found in

Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

flows at Sunstone Knoll in Millard County, Utah.

Andesine controversy

In the early 2000s, a new variety of red or green gemstone resembling sunstone and known as "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. A Tibetan source of bona fide (untreated) red andesine, however, was eventually verified by a number of independent groups of well-respected gemologists.

Oregon sunstone

A variety known as "Oregon sunstone" is found in Harney County, Oregon and in eastern Lake County north of Plush. Oregon Sunstone contains elemental

. 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. On August 4, 1987, the Oregon State Legislature designated Oregon Sunstone as its state gemstone by joint resolution.

References

External links

Mineralogy Database
* {{Jewellery Feldspar Symbols of Oregon Gemstones