What is Pleochroism?

Pleochroism: An Optical Phenomenon

Pleochroism is an optical phenomenon seen in certain minerals and gemstones. Depending on the angle from which the stone is viewed, different colours or shades of colour can be seen.

This effect occurs because these minerals absorb light differently depending on the direction of the light passing through their crystal structure.

Pleochroism can only happen in minerals whose structure is not the same in every direction. Minerals that crystallise in the cubic system, such as garnet, spinel, fluorite and diamond, have the same optical properties in every direction, so they're never pleochroic.

Minerals from the trigonal, tetragonal and hexagonal crystal systems can show two colours, which is known as dichroism. Those from the orthorhombic, monoclinic and triclinic systems can show three, which is trichroism. The differences are sometimes too subtle to see with the naked eye.

Which Minerals Are Pleochroic?

Although amethyst can exhibit pleochroism, it's fairly weak, often showing subtle shifts between purple and reddish-purple.

Tourmaline and tanzanite exhibit strong pleochroism, as does some apatite.

Unheated tanzanite is trichroic. Blue, violet and burgundy are the colours most often described, although brownish and yellow-green shades may also be seen. Heating removes the unwanted brownish colour and enhances the blue and violet, which is why most tanzanite is heat treated.

The cutter then decides whether the blue or the violet faces up, which is why two tanzanites of similar quality can look quite different.

Iolite is another strongly trichroic stone. A single crystal can look violet-blue from one direction, almost colourless from another and yellowish from a third. An old trade name for iolite is water sapphire.

Kyanite is also pleochroic. This mineral, known for its long, bladed crystals, absorbs light differently depending on the direction it travels through the crystal, so different colours can be seen as the stone is turned.

Pleochroism and Gem Identification

Pleochroism is a helpful tool for identification purposes. Even stones that appear similar in colour can often be distinguished by their pleochroic colours.

Gemmologists use a small handheld instrument called a dichroscope, which shows two of a stone's pleochroic colours side by side. The stone has to be viewed from several directions because the colours only separate when light passes through at the right angle.

Ruby shows purplish red in one window and orange-red in the other. A red garnet or red spinel, neither of which is pleochroic, shows the same colour in both from every direction.

This is one of the quickest ways to tell them apart. Spinel, garnet and ruby have been confused with each other for centuries.

Ruby and spinel form together in marble at Mogok in Myanmar. Both can also be found in the gem gravels of Sri Lanka.

Where the Word Comes From

The word pleochroism comes from the Greek words 'pléōn', meaning 'more', and 'khrôma', meaning 'colour', reflecting the phenomenon of multiple colours being visible in a single crystal.

Article Picture

The fluorapatite in the photo above (courtesy of Stan Celestian) is an extreme example of pleochroism.

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The amethyst geodes and kyanite are from our collection.

Tourmaline (var. Rubellite): Courtesy of Stan Celestian. Garnet: Courtesy of Captain Tenneal. Spinel: Courtesy of Géry Parent.

Ruby: Courtesy of James St. John

The photo in this article and the links for the pop-up photos link to the original image on Flickr. You may only be able to view them if you join. Joining Flickr is free, with no additional charges.