REFLECTION EFFECTS IN GEMS AND STONES
Interference - iridescence and play of colour
Rainbow effects such as those seen in cleavage cracks and the iridescence of labradorite, feldspar and haematite are due to interference as light is reflected by thin layers (films) within the gemstone. In moonstone feldspar, the effect is known as schiller (sheen), adularescence or opalescence and the iridescent colours on the surface of the pearl is called the 'orient of pearl'. As light is reflected, there is interference of the wavelengths. Where they coincide, the colour corresponding to the wavelength may be enhanced, in other places waves may cancel each other out and that colour will no longer be seen.
In opal, interference of light occurs as it passes between the regularly arranged spheres that make up its structure. The size of the spheres and the distance between them affects the amount of dispersion and resultant play of colour; as well as the direction from which the opal is viewed. Small spheres produce only the blues and violets as the opal is turned, while large regularly packed spheres show the full range of rainbow colours.
chatoyancy, asterism, and silk
Other internal reflection effects include chatoyancy, asterism and silk, which are caused by inclusions. Chatoyancy is the cat's-eye effect seen on some gemstones that are cut as a cabochon (polished as a rounded dome) and best seen under a bright light such as a spotlight or torchlight. Light is reflected off a parallel arrangement of elongated or acicular (needle-like) crystal inclusions of minerals such as rutile or tourmaline, fibres or long tube-like cavities. Examples of gemstones that may show a cat's-eye include quartz, beryl, ruby,
SAPPHIRE, and TOURMALINE.
Asterism forms star-stones where there are two or more sets of parallel inclusions instead of a single set. Stars may have four, six, twelve, or even twenty-four arms (rays). Star sapphires and rubies generally have six arms, which lie parallel to the crystallographic axes. Other gemstones that may show stars when cut as cabochon include garnet, quartz, and spinel.
Where the inclusions or cavities are not present in large enough concentrations to form a star, they may be seen as silk, with the light reflected from patches of parallel inclusions, such as can often be seen in sapphires.
