Moonstone gemstones. This orthoclase mineral (potassium aluminium silicate) has been cut and polished to produce semi-precious gemstones. It belongs to the feldspar group of minerals, which are related due to symmetry and composition. Feldspars make up 60% of the earth's outer crust. Moonstone is named due to its moon-like glow, which results from the refraction of light between internal layers before exiting the crystal.