Moonstone cabochons in a jewellers window
Moonstone cabochons in a jewellers window (Photo credit: Wikipedia)

A stone that seems to capture the soft light of the moon: what could be a better gem for a fantasy novel? There’s something romantic about this jewel whose sheen evokes moonlight or captive spirits.

Moonstone is a type of feldspar, a large group of aluminum silicates that compose about half of the earth’s crust and are also found in meteorites and on the moon. Feldspars provide a number of minor gemstones, including sunstone, labradorite, amazonstone and a few others. Like the garnet and tourmaline mineral families, feldspars occur in various solid solution series where, say, you can find a pure potassium member on one end and a potassium-sodium variant on the other and any intermediate mix in-between. The primary moonstone mineral is orthoclase but the sodium rich sister of orthoclase called sanidine also produces moonstones.

As with almost all feldspar, orthoclase is typically opaque, often with extremely large crystals. There are reliable reports of opaque crystals up to 30 feet long and stories, which may have some truth to them, of even larger crystals. Occasionally, crystals are transparent or translucent, although these are much smaller. Clear ones can be cut as a gem, often yellow or pale brown. When minute intergrowths of sodium- and potassium-rich feldspars form, the play of light on the layers can produce adularescence: the moonstone effect.

Orthoclase,moonstone (Photo credit: Wikipedia)

Adularescence (from French after the Adula mountains in Switzerland, related to feldspar deposits there, I believe) is a soft white to bluish sheen. As the stone is turned in the light, the sheen can move around and deepen or vanish. This is a beautiful but subtle affect, appropriate for something named after the moon.

The Roman Pliny the Elder (23-79 CE, killed in the eruption of Vesuvius that created the Pompeian ruins) recorded a stone he called an asterion, which most authorities believe was a moonstone, not a rayed-gem like a star sapphire. He described it as “a colorless stone…[from India] having within it the appearance of a star shining brightly like the full moon.” (Courtesy Glimmerdreams.)

The stones are fairly soft, 6 to 6.5 on the Mohs scale, which means common household dust can scratch it. Those with good adularescence are cut en cabochon, usually as a hemisphere or hemi-oval, but sometimes polished as beads or irregular stones. Quality varies from cracked, cloudy, and barely shimmering to ethereal. Naturally, the best quality material is somewhat rare and expensive. Most of what you’ll find on the low-end is not very attractive so beware of those “great deals” on eBay. Stones can be white, champagne colored, or bluish. Online, you can find other colors, although I’m not sure if they are treated or artificial.

The link to the heavens, captured starlight or moonlight, the magical sheen, all might make this a great jewel for a fantasy novel. It can be used simply as an attractive, non-magical stone (perhaps mistaken as magical) but there’s quite a wide range of fantastic options: from a container for captured light, like Galadriel’s Phial, to a repository for a spirit, soul or ghost. The sheen intensity or glow could also be linked to certain occurrences (time of day, presence of a moon, presence of evil or ghosts, etc.) Perhaps, the stones can be used as a ‘trap’ to catch a soul or ghost, or similarly, hold such a spirit that could be communed with or used to power diabolical magic.