Diffraction
This is the fire possessed by certain stones. It is produced by the decomposition of white light into the colours of the spectrum when it passes through a material that acts as a prism. It is most strikingly seen in diamonds.
Some gemstones have spectacular optical effects. Depending on how these stones are cut, their optical effects can be maximised or diminished. These effects can be due to structural features, flaws or inclusions. Here are some of the best known and most striking optical effects that can be found.
Patina
Surface iridescence of metals caused by external alteration. It is typical of chalcopyrite and bornite.
Iridescence
Interference phenomenon with a rainbow effect that occurs in fissures or exfoliations. This effect is common in iris quartz and topaz.
Play of colours
The appearance of plates that change colour depending on their orientation and angle at which the light strikes them. The colours can vary from violet to red, green or even blue, and all the colours of the rainbow can be seen together in the same gemstone. This optical effect is exclusive to high quality opals, also known as noble opals.
Labradorescence
A greyish, bluish or greenish metallic reflection, characteristic of labradorites.
Opalescence
Turbidity that appears in certain stones with dispersed particles such as opal and milky quartz.
Aventurescence
Glow caused by small plates inside the stones. It can be observed in stones such as aventurine quartz or sunstone.
Adularescence
Bluish or whitish glow that seems to float inside the stone. Typical of moonstone, oligoclases and some albites.
Orient
This is the characteristic glow of pearls, caused by the reflection of light on their surface and by the interference of light on the aragonite layers that constitute the pearl.
Cat's eye
A glow in which a moving line of light is seen on the outside of the stone, due to small tubes or needles oriented in the same direction inside the stone. This effect can be found in quartz and tourmaline, for example.
Asterism
A glow that is reflected on the outside of the stone in the form of a moving star. This effect is caused by needles oriented in two or three directions inside the stone. Stones such as corundum or almandine garnets can have this effect.
