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.