Andalusite has the chemical formula Al2SiO5. It is an aluminium neosilicate mineral. The variety chiastolite contains dark inclusions of carbon or clay which form a checker-board pattern when shown in cross-section. A clear variety first found in Andalusia Spain can be cut into gemstones.

Faceted andalusite stones give a play of red, green, and yellow colors that resembles a muted form of iridescence. Iridescence is the property of certain surfaces that appear to change color as the angle of view or the angle of illumination changes. Soap bubbles, butterfly wings and sea shells are some common examples for iridescence.

But the colors are actually the result of unusually strong pleochroism. Pleochroism is an optical phenomenon in which a substance appears to be different colors when observed at different angles, especially with polarized light.