|Is a Variety of||Feldspar|
|Colors||Colorless, white, gray, reddish, pale yellow|
|Hardness||6 to 6.5|
|Fracture||Conchoidal to uneven|
|Cleavage||Perfect 2 directions; imperfect 1 direction|
|Transparency||Translucent to transparent.|
|Optics||α = 1.577; β = 1.585; γ = 1.590. Biaxial (-); 2V= 70°.|
|Optic Sign||Biaxial -|
These feldspars are rarely encountered in gem form. Their occurrence is widespread throughout the world, in a great variety of rock types and environments, but in most cases transparent crystals are rare.
In many cases faceted gems are identified as a feldspar in the plagioclase series, but the finder does not have the instrumentation needed to pin down the species. This is accomplished by a combination of optical and X-ray analysis. A few plagioclase gems have been well characterized, however, and reported in the literature.
Anorthite is from the Greek words an plus orthos, meaning not straight, because the crystal faces meet at an oblique angle.
Anorthite is the most calcic of the plagioclases, and sometimes makes up a distinctive rock known as anorthosite, which has been extensively studied. Localities for the mineral includePala, California; Grass Valley, Nevada; Italian Mountains, Colorado; Greenland; England; Sweden; Finland; Italy; Sicily; India; and Japan. Anorthite has been cut for collectors but very rarely, and faceted gems are always small. However, a locality on Great Sitkin Island, Alaska has yielded cut gems as large as 8 carats. This pale yellow anorthite may be the largest known.