Understanding Pleochroism
Hold a tanzanite crystal and rotate it slowly. Depending on your angle, you may see blue, then violet-purple, then something brownish. Hold an alexandrite and tilt it: the colour shift you notice is partly caused by pleochroism, not just the lighting change. Iolite, a gem sometimes called the “water sapphire,” is so strongly pleochroic that one direction appears deep violet-blue while the perpendicular direction appears nearly colourless — a contrast so extreme it was once used as a navigational tool by Viking sailors.
Pleochroism is one of the most fascinating and practically important optical properties in gemology. It affects how crystals must be oriented for cutting, determines the face-up colour of the finished gem, and produces some of the most spectacular and confusing colour effects in the gem trade. This article explains the physics behind pleochroism, describes how it manifests in key gem species, and explains why cutters must master it to produce optimal results.
The Physics of Pleochroism
Pleochroism is a property of anisotropic minerals — those that belong to crystal systems other than cubic. In anisotropic minerals, the velocity of light and its interaction with the crystal’s electronic structure differs depending on the direction of travel and the orientation of the light’s vibration (its polarisation direction).
Light is a transverse electromagnetic wave, meaning it vibrates perpendicular to its direction of travel. When unpolarised light enters a pleochroic gem, it is split into two rays vibrating in perpendicular planes (this is birefringence). If the crystal absorbs different wavelengths for the two vibration directions, the two rays will appear different colours. When you look at a pleochroic gem from different angles, you change which combination of rays reaches your eye, so the apparent colour changes.
The number of distinct colours observable depends on the crystal system. Uniaxial minerals (hexagonal and trigonal systems, including corundum and tourmaline) have two distinct optical directions and can show two different colours — they are dichroic. Biaxial minerals (orthorhombic, monoclinic, and triclinic systems, including tanzanite, alexandrite, and iolite) have three distinct optical directions and can show three different colours — they are trichroic.
Dichroic Gems: Two Faces
Corundum: The Cutter’s Challenge
Ruby and sapphire are strongly dichroic. In ruby, the two directions show different shades and intensities of red: one direction shows a more pure red, the other shows an orangey-red. To maximise the desirable pure red face-up, the cutter must orient the table facet so that the viewer looks along the direction showing the best colour — which in corundum is perpendicular to the c-axis (looking down the barrel of the crystal, not along its length).
In blue sapphire, the two pleochroic colours are typically blue and blue-green (or blue-violet in some stones). The most desirable blue should face up in the finished stone. For padparadscha sapphire — the rare pink-orange variety prized for its lotus flower colour — pleochroism interacts with the mixed colouring from both chromium and iron to produce complex orientation decisions.
Tourmaline: Dramatic Contrast
Tourmaline is often strongly dichroic, and in some specimens the contrast between the two directions is extreme. A green tourmaline crystal may show vivid green in one direction and near-brown or very dark green in the perpendicular direction. An indicolite (blue tourmaline) may show blue in one direction and very dark blue-green approaching black in the other.
This strong pleochroism means the orientation of tourmaline cuts is critical. Tourmaline is traditionally cut with the table perpendicular to the c-axis (showing the most desirable colour through the table facet) rather than parallel to it (which would produce the darker, less desirable pleochroic colour face-up). The elongated shape of most tourmaline cuts reflects the elongated prismatic crystal habit, which gives the cutter limited options for orientation while maximising weight retention.
Trichroic Gems: Three Directions
Tanzanite: The Most Spectacular Trichroism
Tanzanite is arguably the most spectacularly trichroic gem in commercial trade. Its three pleochroic directions show: violet-blue (the most desirable direction for most cuts), purple or violet, and brown/burgundy. The differences between these three colours in fine tanzanite can be dramatic — a deep royal blue in one direction and a warm brownish-burgundy in the third direction, with purple in between.
This trichroism is commercially leveraged in two ways. First, the cutter orients the table to show blue-violet face-up, which is the most commercially desirable colour. Second, tanzanite is often heat treated to reduce or eliminate the brownish third direction, leaving a more uniform blue-violet. The heating is subtle — it reduces the intensity of the brown component — but it significantly improves commercial appeal and is standard practice in the trade.
Iolite: The Viking Navigation Stone
Iolite (gem-quality cordierite) is among the most strongly pleochroic gems known. Its three directions show violet-blue, blue, and near-colourless (or pale yellowish). The contrast between the deep violet-blue and the near-colourless directions is dramatic and unmistakeable.
Historical accounts suggest that Viking navigators used thin slabs of iolite as polarising filters to locate the sun on overcast days — rotating the slab until the transmitted light was brightest indicated the sun’s position. While the extent of this actual use is debated, it illustrates how distinctive iolite’s pleochroism is: extreme enough to have practical polarising function. In modern jewellery use, iolite must be cut with the desirable violet-blue direction facing up or it appears pale and unremarkable.
Alexandrite’s Pleochroic Contribution
Alexandrite is both pleochroic (trichroic, with directions showing green, orange, and purple-red) and colour-change (showing green in daylight and red-purple in incandescent light). The colour-change is the dominant and commercially defining phenomenon, but pleochroism contributes to the specific hues visible. The quality of alexandrite’s colour change depends partly on the balance of pleochroic components — the finest stones show a complete, dramatic change with minimal compromise from the intermediate pleochroic directions.
Detecting Pleochroism: The Dichroscope
A dichroscope is a simple gemological instrument that separates the pleochroic colours of a gem by creating two adjacent windows through which the two vibration directions are visible simultaneously. In a calcite dichroscope, a piece of Iceland spar (optical calcite) splits the transmitted light into two beams side by side. If the gem is pleochroic, the two windows show different colours; if it is isotropic (cubic) or non-pleochroic, both windows show the same colour.
For identification purposes, the presence and character of pleochroism is diagnostic. Strong blue-violet and brown pleochroism indicates tanzanite. Strong green and near-black indicates green tourmaline. The specific combination of pleochroic colours, combined with refractive index measurements, quickly narrows the identity of an unknown coloured stone.
For cutting and orientation purposes, the dichroscope identifies which direction of the rough shows the most desirable colour, guiding how the cutter should orient the stone before faceting. A ruby cutter using a dichroscope can identify the c-axis direction and confirm that the table will be oriented perpendicularly to it, ensuring the best red faces up in the finished stone.
Practical Implications for Jewellery Professionals
Understanding pleochroism has practical implications for how you display and demonstrate gems, how you advise clients on lighting, and how you evaluate cutting quality. A pleochroic gem may show different colours in the warm incandescent lighting of a restaurant versus the neutral daylight equivalent of a jewellery store — partly due to the light source itself and partly due to subtle changes in viewing angle.
When evaluating a coloured stone purchase, always view it from directly above the table, from the side, and at various tilting angles. If the stone shows significantly different or less attractive colour from side angles, this may indicate that the cutter oriented it suboptimally — choosing weight retention over colour optimisation. In fine coloured stones, cutting for colour is the professional approach; weight maximisation at the expense of colour is a compromise that should be reflected in the price.
