Pleochroism

Pleochroism is the property of showing different colours when a stone is viewed along different crystal directions. Sapphire is the everyday case: down one axis a violetish blue, across it a greenish blue, and the cutter's job is to put the violetish blue face-up. Ruby moves between purplish red and orangey red. Tourmaline can change so hard that one direction is nearly black, and iolite was once called dichroite for the same reason. Tanzanite is the textbook three-colour stone, blue, violet and reddish brown from three directions, though heat removes the brown before most tanzanite reaches a shop. Spinel and garnet do not do it at all, which is one of the older tricks for telling spinel from ruby.

In practice

No report grades pleochroism, but it sits behind two things a report does record. It is an identity test, since a dichroscope shows two colours in corundum and one in glass or spinel. And it explains why orientation is a valued skill and poorly oriented rough sells at a discount: a sapphire cut with its table across the wrong axis will look greenish or grey face-up however fine the material, and no amount of polish rescues it. Dealers in Colombo and Chanthaburi buy rough by holding it to a lamp and turning it, which is pleochroism being priced by eye, and quickly.

On a finished stone, rock it under a lamp and again in daylight. Colour that shifts oddly across the face, or a grey or green tinge that arrives when the stone tilts, is often pleochroism showing through the cut. That earns a question about how the rough was oriented and whether a recut would help. It applies most to sapphire, ruby, tourmaline and tanzanite; a spinel that changes colour on tilting is showing you something else.