Andesine

Andesine is widespread in igneous rocks of intermediate silica content such as andesites and syenites, and occurs in some metamorphic and sedimentary rocks; its first recorded locality is the Marmato district in Colombia. Large amounts of near-colorless to pale yellow gem feldspar are mined in Inner Mongolia, China. Red feldspar has been said to come from the Democratic Republic of the Congo, though that mine was never validated, and from Tibet, where a mine's authenticity has been questioned and a 2010 study judged red andesine labeled as Tibetan most probably copper-diffused.

Andesine lies between oligoclase and labradorite, and its refractive index and other properties fall between theirs, so the boundary is set by chemical composition rather than appearance. Red andesine needs laboratory testing: researchers found traces of copper-bearing flux on rough stones, and argon isotope measurements showed recent high-temperature heating consistent with copper diffusion; in immersion, a green center with a red outer zone may suggest a copper-diffused stone. A 2010 study concluded that red andesine sold as Tibetan or Mongolian is most probably copper-diffused, and that at that time the only reliable source of naturally colored andesine/labradorite was Oregon. Orange-red andesine without glittering inclusions should not be called sunstone.

Andesine is named for the Andes Mountains of South America, where it is abundant in the lavas. Red feldspar said to be from the Congo appeared on the gem market in 2002; at the 2005 Tucson gem shows many red feldspars were marketed as "Chinese andesine", and similar stones appeared the next year as "Tibet sunstone". Red andesine was also promoted as an official gemstone of the 2008 Beijing Olympic Games. Concern that the color was treated led, from 2008, to research into copper diffusion by scientists including those at GIA, Caltech and GAAJ.