Dichroism
Dichroism
Two-colour pleochroism in uniaxial gemstones
Dichroism is the form of pleochroism displayed by uniaxial gemstones — those crystallising in the tetragonal, hexagonal, or trigonal systems — in which two distinct body colours are visible when the stone is examined along different crystallographic directions. The phenomenon arises because optically anisotropic crystals absorb polarised light selectively: light vibrating parallel to the optic axis encounters a different absorption spectrum from light vibrating perpendicular to it, producing two characteristic colours that may differ subtly or dramatically. Dichroism is a reliable diagnostic property and, in many species, a direct guide to optimal cutting orientation.
Physical basis
In a uniaxial crystal, there is a single optic axis (the c-axis in most descriptions) along which light travels without experiencing double refraction. Light propagating in any other direction is split into two rays — the ordinary ray and the extraordinary ray — each vibrating in a plane at right angles to the other and each subject to its own absorption characteristics. The colour perceived by the eye along any given viewing direction is the combined result of which wavelengths survive that selective absorption. Because the ordinary and extraordinary rays have different absorption spectra, the two principal colours of a dichroic stone can range from a near-imperceptible shift in saturation to a complete change of hue.
Distinction from trichroism
Biaxial gemstones — those belonging to the orthorhombic, monoclinic, or triclinic systems — possess three principal optical directions, each with its own absorption spectrum, and therefore exhibit up to three distinct colours: a property termed trichroism. Dichroism is strictly the two-colour equivalent, confined to uniaxial species. Cubic (isometric) gemstones are optically isotropic and show no pleochroism of any kind.
Observation with the dichroscope
The standard instrument for detecting and characterising dichroism is the dichroscope, which exists in two principal forms: the calcite (Iceland spar) dichroscope and the polarising dichroscope. The calcite type splits transmitted light into two adjacent beams polarised at 90° to each other, allowing both principal colours to be viewed simultaneously through a small eyepiece window. To use it correctly, the examiner rotates the stone slowly while directing a strong, diffuse light source through it; the two colour windows are compared at each orientation. A change in the colours seen as the stone is rotated confirms pleochroism; the number of distinct colour combinations observed (two for uniaxial stones, up to three for biaxial) establishes whether the stone is dichroic or trichroic.
Notable examples
- Sapphire (corundum, trigonal): One of the most instructive examples — blue sapphire typically shows a strong blue to violet-blue along one direction and a paler blue to greenish-blue along the other. The cutter's standard practice of orienting the table perpendicular to the c-axis maximises the desirable blue tone visible face-up.
- Ruby (corundum, trigonal): Exhibits purplish-red to orangey-red dichroism; the deeper red is seen perpendicular to the optic axis, again influencing the preferred cutting orientation.
- Morganite (beryl, hexagonal): Shows pink to colourless or very pale pink dichroism; the stronger colour is perpendicular to the c-axis.
- Peridot (olivine — though olivine is biaxial, peridot is commonly cited in introductory dichroism discussions; strictly it is trichroic): Exhibits yellow-green, green, and colourless in its three axes.
- Tourmaline (trigonal): Can display dramatic dichroism — dark green or near-black along the c-axis versus lighter green or yellow perpendicular to it, a contrast so pronounced that cutters routinely orient the table perpendicular to the c-axis to avoid an overly dark stone.
Importance in gem identification and cutting
Because dichroism is an intrinsic optical property determined by crystal structure and chemistry, its presence, absence, and character are valuable identification criteria. A strongly dichroic blue stone is unlikely to be glass or a cubic simulant; the specific colour pair observed can help distinguish sapphire from blue spinel (isotropic, no pleochroism) or from tanzanite (trichroic, biaxial). Gemmological laboratories routinely note pleochroism in identification reports as a supporting observation alongside refractive index, specific gravity, and spectroscopic data.
For the lapidary, dichroism is not merely a curiosity but a practical constraint. Cutting a strongly dichroic stone with the table parallel to the optic axis will present the face-up viewer with a blend of both colours, potentially muddying the hue. Orienting the table perpendicular to the optic axis, by contrast, allows the more desirable of the two colours to dominate the face-up appearance. In commercial practice this consideration directly affects yield decisions and the final value of the finished stone.