Pleochroism simulator

One stone, several colours

Pleochroism is one of the most disconcerting optical phenomena in the mineral world: some gems display different colours depending on the angle from which you look at them. It is neither a reflection nor an illusion: it is the stone's very colour that changes as you turn it. Our simulator lets you “rotate” five pleochroic gems and watch their hues slide from one axis to another.

Why some stones do this

The explanation lies in the crystal structure. In so-called anisotropic minerals, light does not travel at the same speed in every direction of the crystal: it splits, and each component is absorbed differently. As a result, the eye perceives a different colour along each axis. Isotropic minerals — the cubic ones such as diamond, garnet and fluorite — never show pleochroism, and neither do amorphous materials (opal, obsidian, glass).

We speak of dichroism when there are two colours (tetragonal, hexagonal and trigonal systems) and trichroism when there are three (orthorhombic, monoclinic and triclinic systems).

The stars of the genre

Tanzanite is the absolute star: it moves from sapphire blue to violet and on to burgundy — a trichroism so marked that the lapidary must choose, when cutting, which colour to favour. Iolite is spectacular too: blue-violet on one axis, straw yellow on another, nearly colourless on the third. The Vikings are said to have used it as a solar compass under overcast skies. Andalusite alternates yellow-green, orange and red-brown; kunzite, soft pink and violet.

A gemmologist's tool

Beyond the spectacle, pleochroism is a formidable identification test. Gemmologists use a dichroscope — a small tube costing a few tens of pounds — to observe two axes at once. A telling example: a red stone with strong dichroism is probably a ruby; if it shows no pleochroism at all, it is a garnet or glass, both of which are isotropic. The test takes two seconds and settles the matter.