Pleochroism

Pleochroism is the optical property of showing different colors when viewed from different crystallographic directions in a gemstone. The phenomenon is caused by directional differences in how the crystal lattice absorbs light — a single colored stone can have two or three distinct "color axes" that present different hues when the stone is rotated through 90 degrees. Strong pleochroism is a fingerprint of many colored stones (tanzanite, iolite, tourmaline, kunzite); weak or absent pleochroism is the default for high-symmetry minerals (garnet, spinel) and for diamond.

Two-color pleochroism is called dichroism (specifically uniaxial dichroism in crystals with one optic axis — tourmaline, ruby, sapphire). Three-color pleochroism is called trichroism (in biaxial crystals with two optic axes — tanzanite, iolite, andalusite). The distinction is mineralogical: the host crystal's symmetry determines whether two or three axes are possible.

Tanzanite is the textbook trichroic stone. Untreated tanzanite shows blue, violet, and a yellowish or brownish color along its three crystal axes; heat treatment (around 600 degrees Celsius) removes the brownish axis and intensifies the blue-violet pair, which is why nearly all commercial tanzanite is heat-treated. The cutter's choice of orientation determines which axes face up; tanzanite is typically cut to show the strongest blue or strongest violet face-up.

Iolite shows extreme trichroism — viewed along one axis it appears deep violet-blue (the "water sapphire" face), along another it appears yellowish or colorless, along the third it appears a different blue. The cutter must orient the rough carefully to present the strongest violet-blue face-up; an off-axis cut produces a stone that shifts color as the wearer's hand moves.

Tourmaline (especially watermelon tourmaline and bicolor tourmaline) shows pronounced dichroism. The blue-green color in face-up indicolite tourmaline often shifts to yellow-green or yellow when viewed from the side. Pink tourmaline shifts toward red or peach along the second axis.

Sapphire and ruby show dichroism that the cutter manages by orientation. Mogok ruby cut along the optimal axis presents pigeon-blood red face-up; off-axis the same crystal can present a brownish-red. Cornflower blue Kashmir sapphire is cut to present the bluest possible face-up axis; the second axis shows a paler, more greenish blue.

Diamond shows no pleochroism — diamond's cubic crystal symmetry means all directions are optically equivalent and the same color appears from every angle. This is one of the routes a gemologist can use to rule out diamond simulants like cubic zirconia (also non-pleochroic) vs colored stones (almost always pleochroic).

Pleochroism is observed with a dichroscope, a simple instrument with a calcite prism that splits light into two perpendicular polarisations; rotating the stone in front of the dichroscope reveals the color axes directly.