Feldspar.

The feldspars are a large group of aluminium-bearing silicate minerals and, after quartz, the most common ingredient in the earth's crust. They split into two main branches. The plagioclase feldspars run from albite (sodium-rich) through to anorthite (calcium-rich), and include labradorite and sunstone. The alkali (or potassium) feldspars include orthoclase, which is the parent species of moonstone, and microcline, which gives us amazonite.

Several of the most striking optical effects in crystal work come from this family. Labradorite shows labradorescence, the flashes of blue, green, and gold that appear when light hits the layered structure at the right angle. Moonstone shows adularescence, a soft floating sheen. Sunstone shows aventurescence, a glittering shimmer from tiny copper or hematite plates trapped inside.

Feldspars sit around 6 to 6.5 on the Mohs scale, softer than quartz. They tend to have good cleavage, which means they can chip along internal planes if knocked the wrong way. A feldspar piece is best treated like a favourite watch: worn often, kept apart from harder stones, and wiped rather than soaked.