Labradorite

Held at the wrong angle, labradorite is a grey rock. Turn it a few degrees and it throws blue, gold and green. Nothing about the stone changed; you did.

What causes labradorite's flash of colour?

Labradorite is a plagioclase feldspar that separated, as it cooled, into extremely fine alternating layers of slightly different composition. Light entering the stone reflects off those internal boundaries and interferes with itself, and specific wavelengths come back amplified. The result is not a reflection sitting on the surface but a sheet of colour that appears to hang inside the stone and switch on at a particular angle.

Because it is an interference effect, it is directional by nature. There is exactly one orientation where the flash fills the stone.

Why does the cut matter so much for labradorite?

A cutter working labradorite is not shaping for brilliance — they are hunting for the plane where the layers sit parallel to the finished surface, then committing to it. Cut a few degrees off and the colour appears only at the edge, or in a narrow band, or not at all. A great deal of usable rough is ruined this way, and it is why well-oriented labradorite costs disproportionately more than the raw material suggests.

When you are judging a piece, tilt it slowly through a wide arc. A good stone lights across its whole face and holds the flash through a generous range of angles. A poor one flashes once, narrowly, and goes grey.

What are spectrolite and rainbow moonstone?

Spectrolite is a trade name for exceptional Finnish material showing the full spectrum rather than the usual blue. And the stone sold as rainbow moonstone is, mineralogically, a white labradorite — same family, same effect, different body colour.

Is labradorite durable enough for daily wear?

Mohs 6 to 6.5, with the same cleavage as other feldspars. It will not scratch easily in ordinary wear, but it can split if struck squarely. Bezel settings suit it; exposed corners do not.