Trichroism

Trichroism is the specific case of pleochroism in biaxial crystals — crystals with two optic axes — in which three distinct colors appear along three crystallographic directions. The phenomenon is characteristic of tanzanite (the textbook example), iolite, andalusite, and a smaller group of less-traded colored stones. Trichroism is observed using a dichroscope: rotating the stone through three perpendicular orientations reveals the three colors successively.

Tanzanite is the most economically important trichroic stone. Untreated tanzanite (zoisite of gem quality from the Merelani Hills, Tanzania — the only commercial source) shows three colors: blue, violet, and a yellowish-brown or greenish-brown. The three colors all coexist within a single rough crystal; the cutter sees them by rotating the rough through perpendicular axes. Heat treatment at approximately 600 degrees Celsius removes the brownish axis (which is caused by trace vanadium in a specific oxidation state) and intensifies the blue-violet pair. Nearly all commercial tanzanite has been heat-treated; the disclosure is universal and the trade does not discount for it.

Iolite (gem-quality cordierite) shows extreme trichroism. Viewed along the three perpendicular axes, iolite presents deep violet-blue (the "water sapphire" face that gives iolite its alternative trade name), pale yellow or colorless, and a different shade of blue. The cutter must orient iolite rough carefully — an off-axis cut produces a stone that flashes between violet-blue and yellow as the wearer moves, which the trade considers a defect at retail. Properly cut iolite shows the deepest violet-blue face-up consistently.

Andalusite shows what the trade calls "play of color" — the trichroism produces visible color shifts (yellow, green, red-brown) as the stone tilts, which buyers either appreciate as a feature or reject as instability. Top-quality andalusite is cut to balance the three colors so each face-up angle shows pleasing color; lower-quality andalusite is cut for the green face-up only and presents jarring red-brown shifts from the side.

The "trichroic" designation distinguishes biaxial crystals from uniaxial crystals (which show dichroism — two colors). The crystallographic distinction is rigorous: biaxial crystals (orthorhombic, monoclinic, triclinic symmetry) have two optic axes and therefore three independent absorption directions; uniaxial crystals (tetragonal, hexagonal symmetry) have one optic axis and only two independent absorption directions.

Trichroism does not appear on standard lab reports as a graded property; it is observed during identification and noted in descriptive remarks for stones where the phenomenon is dramatic or where the orientation affects appearance materially.