A number of gemstones can be identified by internal optical effects such as iridescence, chatoyancy, asterism, opalescence, play-of-colour, aventurescence, adularescence and orient. These optical effects are grouped under the umbrella of ‘sheen’ and are caused by reflection of light from inside the stone. The light might be reflected from an inclusion in the stone or from the internal structure of the stone.
Let’s look at a few examples of sheen that are diagnostic when seen in a gemstone. The first is orient and is associated with mother of pearl. Mother-of-pearl is the inner layer of a pearl-bearing mollusc which is composed of ‘minute, overlapping platelets of calcium carbonate in the form of aragonite, which allows incidental light to be diffracted, giving the layer its unique appearance’, (p143, ‘Gems and Ornamental Materials of Organic Origin’, Maggie Campbell Pedersen). Pearls are formed out of these platelets which results in the nacreous lustre or ‘orient’ of the gem. It is a subtle sheen with a faint rainbow effect.
Another type of sheen is chatoyancy which is an optical effect that looks like a single mobile streak of light within a stone, like a slit cat’s eye. It is caused by light being reflected off parallel inclusions. Chatoyancy can be seen in chrysoberyl and in other gems like moonstones and tiger’s eye quartz. Asterism is a form of chatoyancy created when two or more sets of inclusions cross each other in different directions, with the reflected light forming a star. Generally seen in corundums (rubies and sapphires) and diopside. Gems with chatoyancy are normally cut as cabochons.
Play-of-colour is seen in precious opal. Opals consist of microscopic spheres of amorphous silica gel which are closely packed together into a 3-D lattice. Light reflects off the various spheres with the colour of the light being determined by the size of the spheres. Larger spheres give a red flash and, depending on the angle the stone is held, can go through the whole colour spectrum. Smaller spheres give blue, green and violet flashes.
In the next post, I will talk more about iridescence, and related optical effects.