Gems TV

OPTICAL EFFECTS

The earth consists of elements made up of countless atoms. Most of these atoms are in orderly, solid arrangements. Materials with such orderly atomic arrangements are said to be crystalline and each different atomic arrangement is called a crystal structure (a regular, repeating, three-dimensional arrangement of bonded atoms).

Most gems are crystalline (i.e. they possess a crystal structure). Atoms bond together most efficiently as orderly crystal structures and as a result, automatically try to pack into the most orderly structure possible. Crystalline materials can be either:

Some gems exhibit unusual optical effects known as phenomena. These rare and beautiful effects often add value to gems. The cat’s eye effect, the star effect and the colour change effect are very popular phenomena and are highly coveted. It is the possession of different crystal structures offered by many gemstones that gives them their unique properties. Without these unique properties, our ancestors would have never valued them as unusual, rare, desirable or beautiful and, in turn, without them, our gemstone choices would be very limited.

Adularescence

Moonstone shows a blue-whitish opalescence (sometimes described as a “billowy” light and shimmer) that glides over the surface of the gem. Interference phenomena from the gem’s layered structure are the cause of this effect.

Asterism

Also known as the star effect, this is a reflection effect that appears as two or more intersecting bands of light across the surface of a gem. It is usually created through reflection of light by thin fibrous or needle-like inclusions that lie in various directions. There are 6 ray, 4 ray and, rarely, 12 ray stars. Ruby and Sapphire cabochons can sometimes very effectively show this phenomenon. Gemstones will produce this effect when:

The quality and value of an asteriated gem is judged by:

Asterism (and chatoyancy) is most visible with direct light, such as a fibre optic light, penlight or another single beam of light, including direct sunlight. With diffused illumination, stars and cat’s eyes are not as distinct (often a problem under TV studio lights).

Aventurescence

This is a colourful play of glittering reflections of small, plate or leaf-like metallic inclusions. Gemstones that display this phenomenon include Aventurine (after which the phenomenon is named) and Sunstone.

Chatoyancy

Also known as the cat’s eye effect, this is a reflection effect that appears as a single bright band of light across the surface of a gemstone, similar to the slit eye of a cat. It is caused by the reflection of light by parallel fibres, needles or channels. Gemstones will show a chatoyant reflection when:

One of the most coveted cat’s eye gems is Cat’s Eye Chrysoberyl - so much so, that if you just mention cat’s eye, it is assumed to be in reference to Cat’s Eye Chrysoberyl. All other cat’s eye gems, such as those found in Tourmaline or Tiger’s Eye, typically have an additional designation.

Colour change

Colour change gems show different colours when viewed under two different light sources. This is due to the gem’s absorption of different colours of the spectrum from different light sources. Examples of gemstones that display this phenomenon include Alexandrite, Colour Change Sapphire and Colour Change Garnet. The sensation of colour change in gems depends upon certain basic requirements:

But exactly how is this colour change effect caused? The light that we see mostly appears to be white - the human brain perceives it as a single colour. However, through science we know that white light is made up of the individual colours of the spectrum; its components are combinations of red, orange, yellow, green, blue and violet light. Lights from different sources have different combinations or balances of these component colours. For example, pure bright sunlight has very strong blue components, while electric light is far richer in the red wavelengths, although it appears very similar to sunlight to our eyes. A small difference in the source of light can sometimes produce a very large difference in our perception of a gem’s colour.

The colour change effect or “Alexandrite Effect” is a rare, beautiful and desirable property in gemstones. When light enters a gemstone it is usually white light. As the light passes through the gem, it absorbs some of the component colours of the spectrum. The resulting mixture of light that is “transmitted” to the human eye has been modified by the gem. The remaining mixture of wavelengths is “added up” by the brain to perceive a single colour. This absorption of certain colours, or wavelengths, is called the “selective absorption of light”. This selective absorption of light wavelengths is always consistent for an individual gemstone. It is this consistency to absorb specific wavelengths that causes our perception of the colour change effect when viewing a gem under two different light sources.

Apart from the standard factors used to assess gemstones, the quality and value of a colour change gem is judged by:

Double refraction & pleochroism

This is an optical “doubling” effect possessed by some gemstones (e.g. Zircon). In these gems we see a twin image of features in the gem. While this effect lends optical depth to a gem like a mirror maze, drawing you in, it doesn’t make a gem more brilliant. Double refraction has one very unusual side effect that greatly intrigued early man and even today some people still find fascinating - pleochroism.

Atoms in some gemstones are arranged in such a manner that light rays are split into two separate components. As these two rays possess slightly different colours, the effect to the eye is that different colours are seen from a gem when viewed from different angles. This body colour property is known as “pleochroism”.

When cutting most pleochroic gems (e.g. Kunzite), lapidaries try to minimise the pleochroism and maximise the single best colour. Andalusite is the opposite, as cutters try to orient the gem to get a pleasing mix of oranges, chocolates, yellows and greens.

Many gemstones are pleochroic, but the two component colours seen by the eyes are so similar that the pleochroism is not particularly visible. Examples of weak to medium pleochroic gems are Ruby, Sapphire, Emerald and Chrysoberyl.

Due to their crystal structure, some gemstones do not possess pleochroism. This lack of pleochroism is extremely useful for species determination. For example, Ruby and Red Spinel share many similar characteristics and one way of distinguishing between the two are by pleochroic tests. Notable examples of non-pleochroic gems are Spinel, Garnet and Diamond.

Iridescence

This is the rainbow-like colour effect seen in some gems and is caused by cracks or structural layers breaking up light into spectral colours. Fire Agate is a gemstone that shows this phenomenon to good effect. When iridescence occurs in metallic hues (called schiller) in Labradorite, it is commonly called “labradorescence”. In Pearls, the subtle iridescence present is called the “orient”.

Play of Colour

These are flashes of rainbow colours in Opal that change with the angle of observation. This should not be confused with “opalescence”, which is the milky blue or pearly appearance of Opal caused by the reflection of light.