From the perspective of the gemstone world, it would seem like quartz is the most abundant mineral in the world. The many quartz varieties include citrine, amethyst, ametrine, chalcedony, aventurine, onyx, agate, tiger's eye, smoky quartz and rose quartz.
But from the point of view of mineralogy, it is feldspar that is the most common mineral. In fact feldspar makes up nearly 60% of the earth's crust. Feldspar is a group of minerals distinguished by the presence of aluminum and the silica ion in their chemistry. This group includes aluminum silicates of sodium oxide, potassium oxide, or calcium oxide.
The name feldspar comes from the fact that some early specimens were found in fields. The term spar is a generic term used by geologists to refer to any non-metallic mineral with a vitreous luster that breaks on distinct planes. The mineral was officially given its name by Johan Gottschalk Wallerius in 1747.
There are two main subgroups of feldspar that produce gem-quality material: the potassium feldspars and the plagioclases, a series that ranges from calcium to sodium feldspars. Among the well-known feldspar gemstones are moonstone, orthoclase, amazonite, andesine, labradorite and sunstone.
Amazonite, moonstone and orthoclase are all potassium feldspars. They have a hardness of 6 to 6.5 on the Mohs scale, and a vitreous luster. Moonstone also exhibits a unique shimmer known as adularescence.
Labradorite, andesine and sunstone are plagioclases feldspars. Like the potassium feldspars, they have a hardness of 6 to 6.5, but they tend to have slightly higher refractive indices, ranking between beryl and quartz.
Apart from gemstones, feldspar is an important industrial mineral. It is a common raw material in the production of ceramics and geopolymers, and is often used as an anti-caking agent in powdered forms of non-dairy creamer. Feldspar is an important ingredient in the manufacture of glass. The alkali content in feldspar acts as a flux, lowering the glass batch melting temperature and reducing production costs.
Granite, an important building material, contains up to 50% to 70% of alkaline feldspar. Feldspars are generally light-colored, including white, pink, tan, green, or gray. The color varies due to impurities within the crystal structure. Feldspar is the mineral that gives granite its pink, green or gray color.