Optical Phenomena in Gemstones

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, and asterism.
'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.

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.

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 is another optical effect that can be seen in aventurine and sunstone.

Asterism produces a four or six-rayed star in some cabochon-shaped gemstones, caused by light reflecting off crystal inclusions of the mineral rutile.  

Tigers 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. 

Pleochroism is an optical effect whereby a stone can display different colours when viewed from different angles.

The one characteristic that all optical phenomena have in common is that they're caused by how light interacts with the stone or its crystalline structure. The extent to which the phenomenon can be seen depends on the angle of light and the angle from which the gemstone is viewed.

Article Photos

The precious opal in our second photo is courtesy of Stan Celestian.