Pleochroism — Different Colours Along Different Crystal Directions
Pleochroism — Different Colours Along Different Crystal Directions
Selective absorption that defines tanzanite, andalusite, and the cutter's orientation problem
Pleochroism is the optical property of anisotropic gemstones in which different colours are observed depending on the crystallographic direction along which the stone is viewed, caused by selective absorption of light wavelengths that varies with the vibration direction of the light within the crystal. The phenomenon occurs only in non-cubic crystals — tetragonal, hexagonal, trigonal, orthorhombic, monoclinic, and triclinic — and is one of the basic optical tests by which the gemmologist sorts species. For the cutter, pleochroism is a problem of orientation: the stone must be cut so that the desirable pleochroic colour faces up.
Dichroism and trichroism
Pleochroism is described as dichroism in uniaxial crystals — tetragonal, hexagonal, and trigonal — where two different colours can be observed corresponding to the two principal vibration directions: light vibrating perpendicular to the optic axis and light vibrating along it. Examples include corundum, where the dichroic colours are typically a deeper and a lighter tone of the body colour, and zircon, where dichroism is generally weak.
Trichroism occurs in biaxial crystals — orthorhombic, monoclinic, and triclinic — where three different colours can be observed corresponding to three principal vibration directions. The three colours represent the maximum range of selective absorption available within the crystal. Examples include tanzanite (zoisite), with blue, violet, and burgundy pleochroic colours; andalusite, with yellow-green, red-brown, and yellow; and iolite (cordierite), with blue, violet-blue, and pale yellow.
Cutting and orientation
Strong pleochroism is the cutter's central problem in many species. The rough must be oriented so that the most desirable pleochroic colour is observed face-up through the table of the finished stone. For tanzanite, the blue-violet face-up orientation requires sacrificing yield from the available crystal because the alternative orientations show less desirable burgundy or violet. For andalusite, designers exploit the strong pleochroism deliberately by orienting the stone to show two contrasting colours simultaneously through different facets — a feature unique to the species in commercial use.
Weak pleochroism, as in most quartz and most diamond colour, has minimal effect on cutting orientation and finished appearance. Stones with weak pleochroism can be cut for yield with little optical penalty.
Observation
Pleochroism is observed with a dichroscope — a small calcite-prism instrument that displays two views of the stone simultaneously, each polarised along a different direction. Rotating the stone through 90 degrees reveals the strongest pleochroic colour pair. For trichroic stones, examination through several crystallographic directions reveals all three colours.
The dichroscope is one of the standard tools of the gemmologist alongside the refractometer, polariscope, and loupe. Pleochroic colour pairs are diagnostic for some species and are recorded in laboratory reports for stones where pleochroism is a key identification or quality factor.
In the trade
For pleochroic species, buying decisions should consider the orientation of the stone as well as the body colour. A tanzanite with a clean violet-blue face-up colour is more valuable than the same rough cut for yield with the burgundy direction emphasised. For andalusite, deliberate use of pleochroism is a positive feature. For sapphire, ruby, and most other corundum, the cutter's orientation choice is part of the workmanship that the buyer is paying for.