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Feldspar is a group of aluminosilicate minerals that contain potassium, sodium, calcium, or barium. It is the most abundant mineral group in the Earth's crust, making up about 60% of continental crust. Feldspars are found in a wide variety of igneous, metamorphic, and sedimentary rocks.

Feldspars are typically hard, vitreous, and have a Mohs hardness of 6 to 6.5. They are generally white or pinkish, but can also be gray, brown, or greenish. Feldspars are characterized by their two cleavage directions that are nearly at right angles to each other.

The two main groups of feldspar are alkali feldspars and plagioclase feldspars. Alkali feldspars are rich in potassium and sodium, and include minerals such as orthoclase, microcline, and albite. Plagioclase feldspars are a solid solution series ranging from albite (sodium-rich) to anorthite (calcium-rich).

Feldspars have a wide range of industrial applications. They are used in the production of glass, ceramics, and as fillers in paints, plastics, and rubber. In jewelry, feldspar is sometimes cut and polished, with notable examples including moonstone and sunstone, which are varieties of plagioclase feldspar exhibiting adularescence and aventurescence respectively. The color and optical properties of these varieties make them attractive gemstones.