4. Optical traits of gems:
Optical traits of natural gems are many, and most have quite technical clarification. But what we need to know at the beginning is about the most important of those properties, and to understand about it in an obvious, straightforward way.
Colour – Colour is the most important optical feature, and quite obvious to the human eye. Very important skill of old and knowledgeable lapidary craftsmen was to achieve a goal: to catch the light in the right way. Which colour we see in a gem depends on what part of light spectrum is refracted, and which part is absorbed while going through the crystal structure of the mineral. The effect of this ratio is the colour of the stone. (This is why, my colleague, the poet author in Rubycharm Wolfgang Whael, often says that “diamonds are made of pure light”.)
From this mineral feature several other optical qualities derive:
The origin of the colour:
Idiochromatic minerals are considered to be those which colour is formed by a presence of some material that are part of the very structure of the gem (i.e. Malachite Green);
Allochromatic are minerals to which colour derives from the occurrence of materials that are not part of the main minerals, and can be considered chemical impurities that occur in regular form (i.e. purple of Amethyst)
Is the gem multicoloured:
Pleochroism – if the colour varies according to the angle we are looking the gem, (or more precisely, if there is diverse light refraction in according to the viewing angle) (i.e. Iolite or Labradorite).
The refractive index is the ratio of light speed and angle alteration that happens after the light went through the gem.
Lustre is the evaluation of the way that light reflects on the surface of the gem. There are basically two main types of lustre: metallic and non-metallic. Metallic lustre is common for precious metals, but also gem mineral called Pyrite, and non-metallic is common for gems. There are several types of known lustre, and most names are telling their own story:
Waxy lustre (Turquoise), Pearly, Resinous (like resins – Amber), Silky (Selenite), Earthly, Greasy (Jade), Adamantine (Diamond) and Vitreous (glass-like, Obsidian).
There are some other special optical effects caused by particular qualities such as fire, adularescence, cat’s eye, star effect…