The feldspars are a large group of aluminum-containing, monoclinic or triclinic silicates that comprise the majority of the earth’s crust. They are classified by way of the relative quantities of potassium, calcium and/or sodium that are present within their respective compositions, with the primary divide occurring between the calcium-containing plagioclase feldspars (i.e. anorthite, bytownite, labradorite, andesine and oligoclase) and the potassium-containing alkali or k-feldspars (orthoclase, sanidine and anorthoclase). However, albite - which is the sodium-dominant member of the group - also possesses small amounts of both calcium and potassium, and therefore straddles both classifications.
The most significant feldspars within the jewelry trade are labradorite, andesine, oligoclase, albite and orthoclase, with the latter two combining to form the well-known gem variety, moonstone. In addition, the presence of tiny, plate-like inclusions of copper, goethite or hematite within oligoclase or, more rarely, orthoclase gives rise to the spangled effect of sunstone.