The 4 Cs: Colour

Posted on the 10th November 2016

Colour is seen to be one of the most important factors in gemstone materials with the combination of hue, tone and saturation contributing to the overall beauty and value. 

In coloured gemstones the colour is generally the key factor when determining beauty, whereas in diamonds the perceived lack of colour is generally deemed to be most appealing; but both are equally as important.

When we see a coloured gem this is due to the material absorbing certain wavelengths of visible light but not others; different wavelengths of light are responsible for different colours so this absorption in-turn affects the colour we see. For example, a red gemstone such as ruby, will absorb all wavelengths of visible light except red light – so the gem will appear red.

The colour we see in gems is due to the presence of specific elements within the internal crystal structure; this is the case for both coloured stones and diamonds. These elements form either an essential part of the chemical formula, as is the case with peridot or, as is most often the case are present rather as an impurity. Many gem minerals are colourless in their pure form but due to impurities are more often seen as coloured. For example, corundum in its pure form is colourless but if chromium is present we see it as ruby, or iron and titanium we see it as sapphire.

Some coloured gems also have so called phenomenal properties, that is, the light reacts within some gem materials in certain ways so as to produce different effects. Some of these are:

Play of colour: most commonly seen in opal. The prismatic display of colours seen when looking at precious opals is due to the refraction of white light within microscopic spheres in the material. As the light refracts we see a ‘rainbow’ type effect. Some opals will display all colours across the spectrum from red right through to violet, whereas others may only display one, two or three. 

Colour-change effect: some gem materials can display more than one colour when viewed in differing lighting conditions. The best example of this is the colour-change chrysoberyl variety alexandrite. Due to the chemical elements within the material alexandrite appears green in daylight but red in incandescent light. Other gems such as sapphire and garnet can display colour change effects but fine quality alexandrite will show one of the most dramatic colour changes.

Pleochroism: certain gem materials will display multiple colours when viewed from different angles; this is due to light reacting differently in certain directions within the gem material. These can be shades of the same colour, or two or three different colours. The gem iolite exhibits strong pleochroism; although it is seen as a violet gem the pleochroic colours that can be seen are blue, violet and pale yellow.

Many gem materials on the market are subject to different treatments to improve their colour with many of these being widely used and accepted in the industry. 

These treatments, carried out on both diamonds and coloured stones are usually undertaken to lighten or intensify the colour of a gem material, to reduce a certain hue or to produce a completely different colour altogether. Heat treatment is commonly used to improve the blue colour in sapphires and to improve the hue of some rubies by removing the blue tones to leave a more pure red colour. Heat treatment can also be used to alter some amethysts to a golden citrine, and also for aquamarine to reduce the green tones and improve the blue colour.

Irradiation is a common treatment for diamonds – producing a wide range of fancy colours; colourless topaz also commonly has a combination of irradiation and heat treatment to produce a wide range of blue hues. 

Our qualified gemmologists are happy to guide you through choosing the most appropriate gemstone for your jewellery collection.