The most
abundant group of
minerals in the
earth's
crust. Quite a lot of them have been used for
jewelry; most of them are
translucent or
opaque but a few form
transparent crystals. It can take chemical analysis to tell exactly what particular type of feldspar a material is, since they all are in the 6 to 6.5 range on the
Mohs hardness scale and have other characteristics in common.
Feldspars are divided into subfamilies such as the plagioclase (including labradorite, spectrolite) and the orthoclase (including moonstone, adularia) and the oligoclase (including sunstone) families.