Looking inside the Stone: Iridescent sheen

One of the largest groups of gemstones are the feldspars, making up 60% of the earth’s crust. There are a number of gem quality varieties in the feldspar family and many of these varieties display an attractive iridescence sheen, that is, lovely play of colour showing when the stone is turned, due to refraction or internal structures. Moonstone is the best known variety of feldspar to show iridescence. When cut as a cabochon, moonstones, which are usually almost colourless, translucent to transparent stones, have an adularescent sheen which can be a blue or white. Historically, they are linked to worship of the moon as they were seen as reflecting the moon.

Another feldspar variety is labradorite. Generally, labradorite gems are opaque to translucent and can display a lovely metallic iridescence in blues and greens, and some yellows. The iridescence is named labradorescence after the stone itself.

There is an almost transparent colourless labradorite called ‘rainbow moonstone’ which shows a lovely blue and yellow, and sometimes more colours, iridescence.

Another feldspar with iridescence is aventurine feldspar which is also called a sunstone. Aventurescence is a spangled metallic sheen caused by lots of tiny mineral inclusions. It can also be found in aventurine quartz as seen in the earrings below.