Optical Phenomena in Gemstones

Optical phenomena are the visual effects produced when light interacts with a gemstone, its internal structure, or the tiny inclusions within it. They include play of colour in opal, labradorescence in labradorite, the star of asterism, and the moving band of light known as chatoyancy. Where present, they can significantly increase a stone's value.

The Effect of Light on Gemstones

Many gemstones exhibit optical properties. These characteristics, often described as optical phenomena, can lead to a significant increase in value.

Optical phenomena are often caused by the way light interacts with a stone, its internal structure, or inclusions.

The word 'lustre' is used to describe the way light reflects off the surface of a gemstone. Although the type of lustre known as vitreous is one of the most common, there are several others.

Lustre should not be confused with brilliance, which is the reflection of light from within a cut gemstone.

Terms used to describe optical phenomena include play of colour, adularescence, schiller, labradorescence, aventurescence, chatoyancy, asterism, iridescence, dispersion and colour change.

Play of Colour

'Play of colour' in precious opal is caused when light is diffracted as it passes through an orderly arrangement of tiny silica spheres, producing a spectrum of colours.

Labradorescence and Schiller

The optical effect seen in labradorite is caused by light reflecting and interacting with extremely fine structures within the stone, producing flashes of colour that change with the viewing angle. The colours only appear when light interacts with the stone and is reflected back to the eye.

This phenomenon is known as schiller, but the term labradorescence is used when present in labradorite.

Adularescence

The term adularescence describes a different optical effect seen in moonstone, which produces a billowing, milky-white glow or blue sheen that appears to float just beneath the surface.

Aventurescence

Aventurescence is a glittering, metallic sparkle caused by light reflecting off large numbers of tiny, flat inclusions within the stone. It can be seen in aventurine, where the inclusions are usually a green mica called fuchsite, and in sunstone, where they are typically platelets of copper or hematite.

Asterism

Asterism produces a four or six-rayed star in some cabochon-shaped gemstones, caused by light reflecting off needle-like crystal inclusions. These are most often the mineral rutile, although other minerals can produce the same effect. The stone must be cut as a cabochon for the star to appear.

Chatoyancy

Tiger's eye and chrysoberyl are known for exhibiting chatoyance. As light reflects off fibrous inclusions within the stone, it produces a parallel band of light. As the direction of light changes, the band appears to move, giving the impression of movement within the stone.

Iridescence

Iridescence describes rainbow-like colours that change depending on the angle from which a stone is viewed. It's caused by light interfering with thin layers, fractures or films within the stone, and can be seen in materials such as rainbow obsidian and ammolite. Our article explains what iridescence is in more detail.

Dispersion

Dispersion, commonly referred to as 'fire', occurs when white light entering a cut gemstone is separated into the colours of the spectrum. It's the effect responsible for the flashes of colour seen in diamond, and it's even stronger in stones such as sphene and demantoid garnet.

Colour Change

Some gemstones appear to change colour under different lighting. The best known is alexandrite, which can look green in daylight and red under incandescent light. Rather than reflecting light, the stone absorbs certain wavelengths, so the colour we see depends on the light source.

Pleochroism

Pleochroism is an optical effect whereby a stone can display different colours when viewed from different angles. Unlike most of the effects above, it isn't caused by light reflecting off inclusions or structures. Instead, the crystal absorbs light differently depending on the direction in which it passes through, so the colour changes according to the angle you look through the stone. Iolite and tanzanite are well-known examples.

What They Have in Common

Most optical phenomena are caused by how light interacts with the stone, its inclusions, or its crystalline structure, and the extent to which the effect can be seen depends on the angle of the light and the angle from which the gemstone is viewed.

Colour change and pleochroism work differently, as both depend on how light is absorbed rather than reflected. In every case, though, what we see is the result of light and stone interacting, which is why a gemstone can look so different from one moment to the next.