Moonstone Quality Factors
Moonstones range in appearance from semi-transparent to opaque and colourless to white, with a blue, silver or white adularescent effect. Moonstone bodycolours vary widely. They can be green, yellow to brown, or grey to nearly black. Along with adularescence, some moonstones show chatoyancy, also called the cat’s-eye effect. A few show four-rayed stars in an effect called asterism.
Throughout its long history, people have agreed on the qualities that the most highly favoured moonstones should display: a colourless, semitransparent to nearly transparent appearance without visible inclusions, and a vivid blue adularescence, known in the trade as blue sheen. The finest moonstone is a gem of glassy purity with a mobile, electric blue shimmer.
Bodycolour should be nearly colourless and free of any yellowish, brownish or unattractive green tints. Adularescence should, ideally, be blue. The sheen should be centred on the top of a cabochon, and it should be easily seen from a wide range of viewing angles. If a moonstone’s adularescence is only visible within a restricted viewing range, its value drops.
In 1997, miners in Southern India discovered a new type of moonstone with a bright green bodycolour, described as “parrot green” by the trade. Moonstone’s signature adularescence floats in this sea of green. The gem also displays a light yellow pleochroism, a term used for a display of different colours in different crystal directions.
Today’s popular moonstone colour variety includes orange to yellow colours called peach.
A good moonstone should be almost transparent and as free of inclusions as possible. Inclusions can potentially interfere with the adularescence.
Characteristic inclusions in moonstone include tiny tension cracks called centipedes. They are called this because they resemble those long, thin creatures with many legs.
Moonstone might be shaped into beads for strands, but by far the most common cutting style is the cabochon, a form that displays its phenomenal colour or colours to best advantage. Moonstone cabochons are usually oval, but cutters sometimes offer cabochons in interesting shapes, such as the tapered sugarloaf—an angular cabochon with a square base.
If a moonstone is cut as a cabochon, the dimensions should be uniform and the profile shouldn’t be too flat. Very flat cabochons don’t display sheen well and have little value.
Faceted moonstones have become increasingly common. The cut heightens brilliance and tends to hide any inclusions that might be present.
Moonstone is popular for carving into decorative jewellery elements, such as cameos or the popular “man-in-the-moon” face that plays on the gem’s name. The uneven surface of the carving combines with the shifting adularescent sheen to create a delightfully intricate and lively effect.
Moonstone comes in a wide range of sizes and carat weights. Fine-quality material is becoming scarcer in larger sizes.