Understanding Optical Phenomena in Gems

Some gemstones do something beyond simply showing colour. A star sapphire glows with a six-rayed star that floats across its surface as it moves. A cat’s eye chrysoberyl opens and closes its shimmering eye of light. A moonstone pulses with a blue glow that seems to come from deep within. An alexandrite shifts from green to red as it moves from sunlight to incandescent light. These effects — called optical phenomena — are among the most captivating and commercially powerful properties in the gem world.

Understanding the science behind optical phenomena allows jewellery professionals to explain these effects with authority, justify premium prices, and create moments of wonder for clients that translate directly into sales. This article covers the major phenomena encountered in the gem trade: asterism, chatoyancy, adularescence, labradorescence, colour change, iridescence, aventurescence, and play of colour.

Asterism: The Star Effect

Asterism is the appearance of a star-shaped pattern of light on the surface of a cabochon-cut gemstone. The star is formed by the reflection of light from needle-like inclusions (most often rutile) oriented along specific crystallographic planes of the host mineral.

In corundum (ruby and sapphire), rutile needles grow along three sets of planes defined by the trigonal crystal structure, spaced 60 degrees apart. When the stone is cut as a cabochon with the dome perpendicular to the crystal’s c-axis, light reflecting from these three sets of needles creates a six-rayed star. Some rubies and sapphires show a twelve-rayed star, caused by two separate orientations of needles.

Star garnets from Idaho and India show four-rayed or six-rayed stars. Rose quartz can show a diffuse star when cut as a sphere. Star diopside, star enstatite, and star spinel are rarer examples of asterism. The quality of a star is assessed on sharpness, straightness of the rays, centring, and how well the star is visible in low-light conditions.

Fine star rubies and star sapphires from Burma command extraordinary prices. The ideal star is centred on the dome, the rays are straight and well-defined, and the body colour is rich. A common issue is that heat treating corundum to improve colour dissolves the rutile needles, eliminating the star — so untreated star stones are particularly prized.

Chatoyancy: The Cat’s Eye Effect

Chatoyancy (from the French “oeil de chat” — cat’s eye) is a single ray of light that moves across the surface of a cabochon, resembling the vertical pupil of a cat’s eye opening and closing as the stone tilts. It is caused by parallel needle-like or tubular inclusions or growth tubes that reflect light collectively along a single direction.

The finest cat’s eye effect occurs in chrysoberyl cat’s eye from Sri Lanka and Brazil. The best stones show a sharp, bright ray of light precisely centred on the stone, contrasting with a rich honey-yellow or apple-green body colour. When a light source hits a cat’s eye chrysoberyl, the stone should show the “milk and honey” effect: one side of the ray appears whitish and the other side shows the body colour, with the boundary shifting as the stone moves.

Cat’s eye effects can occur in many other species: quartz cat’s eye (less valuable), tourmaline cat’s eye, aquamarine cat’s eye, and moonstone can all show the effect. In the trade, “cat’s eye” without a species qualifier almost always refers to chrysoberyl cat’s eye, which is the standard by which all others are judged.

When selling a cat’s eye, demonstrating the effect properly is essential. The stone must be held under a single, directional light source — not diffuse overhead lighting. Teach clients how to show their stone to best advantage, and they will be delighted every time they demonstrate it to friends.

Adularescence: The Moonstone Glow

Adularescence is the billowing blue or white glow that appears to float just below the surface of moonstone. It is caused by light scattering from alternating layers of different feldspar minerals (typically orthoclase and albite) that form during the cooling of the original melt. These layers are extremely thin — on the order of visible light wavelengths — and scatter specific wavelengths preferentially.

The finest moonstone shows a deep blue adularescence over a colourless body, seen in stones from Sri Lanka (Ceylon moonstone). As layer thickness increases, the scattered light shifts from blue toward white. Stones from India often show a more milky or rainbow adularescence over a body that may be grey, peach, or green.

Rainbow moonstone is actually a variety of labradorite (a different feldspar), not orthoclase moonstone, but it shows a related phenomenon and is commercially sold under the “moonstone” name. True orthoclase moonstone and labradorite moonstone are different minerals with similar market positioning.

Labradorescence: The Labrador Spectacle

Labradorescence is the spectacular display of metallic rainbow colours — blues, greens, golds, oranges, and reds — that appears in labradorite and spectrolite when the stone is moved under light. Like adularescence, it is caused by interference of light in thin alternating layers within the crystal, but the layers in labradorite are thicker and the resulting colours are more dramatic and varied.

The finest spectrolite from Finland shows full-spectrum labradorescence over a very dark body, creating dramatic contrast. Gem-quality labradorite faceted for jewellery shows colour that shifts with viewing angle. Golden labradorite (bytownite) and Oregon sunstone are related feldspars that show different phenomena.

Colour Change: The Alexandrite Effect

Colour change is the property of appearing different colours under different light sources — typically daylight (or fluorescent light, which is rich in blue-green wavelengths) versus incandescent light (rich in red-orange wavelengths). The most famous colour-change gem is alexandrite, which appears green in daylight and red under incandescent light.

Alexandrite’s colour change is caused by chromium absorption. The chromium in alexandrite absorbs strongly in both the blue and yellow regions, transmitting a window of light that straddles the green and red parts of the spectrum. In daylight, the blue-rich illumination emphasises the green; in incandescent light, the red-rich illumination emphasises the red. The human eye’s sensitivity shift between the two conditions amplifies the apparent colour change.

The strength of colour change is the primary quality factor after colour itself. Fine alexandrite should show a complete change — strongly green in daylight, strongly red under incandescent — with minimal grey or brown modifying the colours. Stones with strong colour change from untreated sources (Russia, Sri Lanka, Brazil, East Africa) command extraordinary premiums.

Other colour-change gems include colour-change sapphire (blue to violet or purple), colour-change garnet (some of the most dramatic colour changes known), colour-change spinel, and colour-change fluorite. Synthetic alexandrite is widely available and shows stronger colour change than most natural stones, making disclosure critical when selling.

Play of Colour: Opal’s Fire

Play of colour is the spectral flashing of multiple colours seen in precious opal. Unlike most optical phenomena, play of colour in opal is not caused by inclusions, crystal layers, or trace elements — it is caused by diffraction of light by a three-dimensional grid of silica spheres arranged in a regular array within the opal structure.

When the spacing of the silica spheres is comparable to the wavelength of visible light, the spheres act as a diffraction grating, breaking white light into its spectral components. Different sizes of spheres produce different colours: smaller spheres (around 150 nm) produce violet; larger spheres (around 330 nm) produce red. Opals showing red in their play of colour are the most valuable because large sphere sizes are rarer.

The play of colour in opal is assessed on brightness, range of colours, pattern, and coverage. The most prized opal patterns have specific trade names: harlequin (large angular patches of colour), pinfire (small bright points), and rolling flash (broad sweeping colour). Black opal from Lightning Ridge, Australia, with dark body colour providing maximum contrast for the play of colour, commands the highest prices.

Aventurescence and Iridescence

Aventurescence is the glittery metallic sparkle seen in aventurine quartz (caused by platelets of fuchsite mica) and sunstone (caused by copper or hematite platelets). The flat, reflective platelets act like tiny mirrors, creating a sparkling, reflective shimmer when the stone moves.

Iridescence is a rainbow-like play of surface colours seen on the outer surface of some gems. It occurs in iris quartz (from fine internal fractures), some cultured pearls on their surface nacre, and on the wing-case beetle inclusions found in some amber. Surface iridescence in pearls is called orient, and is one of the quality factors assessed when valuing fine pearls.