The Gemmologist’s Eye

Some stones are valued for a colour that stays still. Moonstone is valued for a light that moves. Tilt one slowly under a single light and a soft sheen rolls across the dome, gathers, and drifts on, like light seen through thin cloud. That sheen is not polish, and it is not a coating. Moonstone…

Pearls are sold beside stones, graded beside stones, and kept in the same boxes as stones. They are not stones. A pearl is grown, not mined. Inside a living mollusc, layer after layer of nacre — microscopic platelets of calcium carbonate bound with organic matter — builds around an irritant or a nucleus. No crystal,…

Some coloured stones look lit from inside. Paraíba tourmaline is the clearest case — and no, it does not glow in the dark. What it does is return the light already in the room so saturated with colour that the eye reads it as a light source rather than a coloured object. The colour comes…

In jewellery, paste means glass — lead glass, cut and set to look like a gemstone. The word now sounds cheap. In the eighteenth century, it did not. Paste was worn openly, at court, by people who owned diamonds as well. The Parisian jeweller Georges Frédéric Strass refined the glass in the 1720s, had it…

Look at a stone from the front and it can seem to be one thing. Look at it from the side and it may tell a different story. A doublet is two pieces joined and sold as one stone. A triplet is three. In an opal doublet, a thin slice of precious opal — the…