Feldspar.

The feldspars are a large group of aluminium-bearing tectosilicate minerals and, after quartz, the most common ingredient in the earth's crust. They make up roughly sixty percent of all crustal rock by volume, which is why feldspar fragments turn up in granite countertops, beach sand, and the pottery glazes that have been fired for thousands of years. The name comes from the German Feldspat, meaning "field stone," a reference to how often farmers turned the white grains up while ploughing.

The two branches

Feldspars split into two main branches, defined by which other element accompanies the silicon, oxygen, and aluminium in the crystal structure. The plagioclase feldspars run as a continuous chemical series from albite (sodium-rich, NaAlSi3O8) through oligoclase, andesine, and labradorite to anorthite (calcium-rich, CaAl2Si2O8). Labradorite, sunstone, and the rare blue moonstone known as rainbow moonstone all sit on this plagioclase branch. The alkali (or potassium) feldspars include orthoclase (KAlSi3O8), which is the parent species of classical moonstone, sanidine (a high-temperature variant), and microcline, the species that gives us green amazonite. Anorthoclase, which sits between the two branches, produces some of the gem-quality moonstones from Madagascar and Sri Lanka.

Several of the most striking optical effects in crystal work come from this family, and most are caused by the way feldspars unmix on cooling into very fine alternating layers. Labradorite shows labradorescence, the flashes of blue, green, and gold that appear when light reflects off submicroscopic layers of differing composition (a structure called a Bøggild intergrowth). Moonstone shows adularescence, a soft floating sheen produced by the scattering of light between alternating orthoclase and albite layers. Sunstone shows aventurescence, a glittering shimmer from tiny copper, hematite, or goethite plates trapped inside during growth. Amazonite owes its blue-green colour to trace lead and structural water, the modern explanation having replaced the older copper hypothesis after work by Hofmeister and Rossman in the 1980s.

Care and the buyer

Feldspars sit around 6 to 6.5 on the Mohs scale, softer than quartz, and orthoclase is in fact the reference mineral for 6 on Friedrich Mohs's original 1812 list. They have good cleavage in two directions meeting at very close to ninety degrees (the name orthoclase literally means "straight break"), which means they can chip along internal planes if knocked the wrong way. The two cleavage angles are slightly different in plagioclase, where they meet at about eighty-six degrees, a feature only really visible under a microscope.

A common misconception is that all moonstones are the same species. They are not. Classical Sri Lankan moonstone is orthoclase. Rainbow moonstone, often sold under that name, is usually an iridescent labradorite. Both are feldspars, but they sit on different branches of the family. Another misconception is that labradorite's flash is some kind of inclusion or surface treatment. It is structural, an internal lighting effect that no surface polish can add or remove.

For everyday wear, a feldspar piece is best treated like a favourite watch: worn often, kept apart from harder stones such as quartz crystals or topaz in the same drawer, and wiped rather than soaked. Avoid ultrasonic cleaners, which can encourage hidden cleavage planes to open, and keep moonstone and amazonite away from prolonged direct sunlight, which can fade the colour of some material over years.