Feldspar is a group of minerals composed of aluminosilicates of potassium, sodium, calcium, or occasionally barium. They occur as single crystals or as masses of crystals and form an important constituent of many igneous rocks including granite, syenite, gneiss, rhyolite, and basalt. Although the feldspar minerals may belong to either the monoclinic or triclinic systems, they nevertheless resemble each other in crystal habit, methods of twinning, and especially having cleavage surfaces inclined to each other at a 90 degree angle, or nearly so. They have a hardness about 6 and a specific gravity ranging from 2.55 to 2.75. Feldspathic minerals have a vitreous luster and vary in color from white or colorless to various shades of pink, yellow, green, and red. All the feldspars weather readily under the action of both atmospheric agencies and acidic surface waters, yielding a clay known as kaolin.

Orthoclase, a monoclinic feldspar, is one of the most common of all minerals. It is often white, gray, or flesh red in color and sometimes occurs as colorless crystals. Orthoclase is used extensively in the manufacture of glass and porcelain. Aldularia is a colorless, translucent to transparent variety of orthoclase.

Microcline, a feldspathic mineral crystallizing in the triclinic system, is identical with orthoclase in chemical composition and virtually identical in physical properties. It occurs occasionally in the form of enormous single crystals. The industrial uses of microcline are similar to those of orthoclase. A green variety of microcline, Amazon stone, is valued as a gem stone when highly polished.

The plagioclase feldspars, which also crystallize in the triclinic system, consist of aluminosilicates of sodium and calcium. Pure sodium aluminosilicate is called albite, and olgioclase, andesine, labrodorite, bytownite, and anorthite are minerals with increasing percentages of calcium, Anorthite is a pure calcium aluminosilicate. The plagioclase feldspars are of lesser commercial importance than orthoclase and microcline. They sometimes show an attractive play of colors and are polished as semi precious stones. Opalescent albite and iridescent laborodite are called moonstones. Olgioclase with included impurities which cause a sparkling effect is called sunstone, or incorrectly adventurine.