The term 'moonstone' refers to a type of gemstone that is composed of two or more minerals, usually adularia, FELDSPAR, and OPAL. Adularia is a type of FELDSPAR that is composed of potassium and aluminum silicate, while FELDSPAR is a group of minerals that contain aluminum, silicon, and oxygen. OPAL is a hydrated amorphous form of silica, which is composed of silicon and oxygen. All three of these minerals are found in moonstones, giving them their unique appearance and properties.