Causes of Colour Color in Gems Gemstones

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This was an assignment I did for my Year 2 Theory in Gemmology (GAA). It seems a shame to waste it so for your interest and education I thought you might like a read...

At first glance the thing you notice about a gem is its colour then other things become apparent. It’s sparkle, sheen, lustre, clarity also come into play…all producing a symphony of delight to the eye.

The clarity of a gem is a fairly obvious feature. Technically known as Diaphaneity, it really means the transparency of a gem and can be boiled down to 3 categories.
  1. Transparent. Perfectly clear, that is, you can see objects through it…gems such as Diamond, Aquamarine, Quartz.
  2. Translucent. Where light passes through the gem but you can’t see through it…Opal, Jade, Chalcedony.
  3. Opaque. Where no light passes through the stone and you only see surface features…Malachite, Turquoise, Lapis Lazuli.
These definitions are on a sliding scale where a gem can be semi-transparent or almost opaque or anywhere in between. Also it doesn’t mean that the gem is colourless like glass, as in the case of a transparent red Ruby and most other gems. It can be any colour; it’s the amount of light passing through that’s important.

Without going into the wave and quantum theories of light….
It is very difficult to talk about colour without saying a few words about light and its nature. Firstly ‘colour’ is the eye’s reaction to a particular wavelength of light received. White light is composed of all the colours of the rainbow (visible wavelengths, red 700nm to blue 400nm) and if a colour is missing from that rainbow, white light changes to a coloured light depending on what is missing. For example if the violet end of a spectrum is absorbed, the colour seen is yellow and vica-versa. No red and you get green, no orange and you get blue. This is known as a complimentary colour and is one of the main ways in which gemstones are coloured and is due to certain elements being present in the gem that absorb a particular wavelength and this is known as selective absorption.
The main elements that cause the majority of colours in gemstones are know as the Transition Elements and are a consecutive group in the Periodic Table They are…Titanium, Vanadium, Chromium, Manganese, Iron, Cobalt, Nickel, and Copper. If an element or combination of elements is present, selective absorption may take place producing a coloured gem. That colour may not be the same for the same element in different gemstones.
  • Titanium colours blue Zoisite and blue Sapphire (with iron) and more.
  • Vanadium colours Grossular garnet, synthetic Sapphire (Alexandrite Var.) and more.
  • Chromium colours Ruby, Emerald, red Spinel and many others. Note… Green / Emerald and Red / Ruby.
  • Manganese colours Rose Quartz, Rhodonite, Andalusite and more.
  • Iron colours Peridot, Aquamarine, Amethyst, blue / green Tourmaline and more.
  • Cobalt colours blue glass and synthetic blue Spinel. Note that it is rarely found in natural gems.
  • Nickel colours Chrysoprase, synthetic green and yellow Sapphires
  • Copper colours Malachite, Turquoise, Diopside and more.
A few other elements have colouring properties in gemstones that are not in the Transition group, notably Selenium is used to make glass red, Praseodymium and Neodymium (collectively Didymium) colours Apatite , Thorium and Uranium colours Zircons, Nitrogen makes Diamonds yellow and Boron makes them blue.

These colouring agents (elements) may be present in the gem in two distinct ways...
Firstly they may be part of the gems integral structure and composition without which they would not be that gem. They are known as idiochromatic gems. Iron in Peridot and Almandine Garnet, Copper in Malachite and Turquoise or Manganese in Rhodonite and Rhodochrosite are all idiochromatic gems.
The second way gemstones are coloured is by the presence of these elements as impurities in small or trace amounts. They are not integral to the gem and without these impurities many gems could be colourless.  The atomic size and charge of these transition elements allows for replacement of some atoms of the gem without affecting the nature or structure of the gem. These are known as allochromatic gems and represent the majority of cases. Quartz is transparent; add a little iron and you have Amethyst. Colourless Sapphire and a bit of iron and titanium and you have blue sapphire or if you add nickel to synthetic corundum, you get green and yellow Sapphires. Chromium makes Rubies red and Emeralds green in this manner.

Most gems are crystalline in nature and many are single crystals. Most crystals have directional physical properties because the crystals structure is different dependent on direction. One of those properties is that they affect the light passing through them with the direction of passage.
  • Isotropic gems (cubic system) have no directional effect on light.
  • Uniaxial gems (tetragonal, hexagonal and trigonal system) have 2 directions in which they change light passing through them.
  • Biaxial gems (orthorhombic, monoclinic and triclinic system) have 3 directions in which they change light passing through them.
This leads to the fact that the colour of the gem will most often change with the direction of view (except for isotropic gems). This is known as differential selective absorption. The term ‘differential’ relates to the direction and ‘selective absorption’ is the same as mentioned previously. This effect is also known as Pleochroism where a gem exhibits 2 or 3 distinct colours.
A few examples in the uniaxial system;
  • Rubies can appear… pale red / deep red,
  • blue Sapphires… deep blue / green blue,
  • Aquamarine… blue / colourless,
  • red Tourmaline… pink / dark red,
  • green Tourmaline… pale green / dark green .
In the biaxial system where there are 3 directions;
  • Andalusite… yellow / green / red (most striking and known as the ‘traffic light’ gem for that reason),
  • Zoisite (Tanzanite)…blue / purple/ green,
  • green Topaz… green / blue green / colourless,
  • blue Topaz…pale pink / blue / colourless.
Nearly all uniaxial and biaxial coloured gems exhibit pleochroism to some extent, from negligible to extreme.

The structure of the gem can affect its colour properties. Sheen is the term used for light that is reflected from below the surface of the gem. Most types of sheen have little effect on colour such as asterism, chatoyance, adularescense however there are a few that do effect the colour of the gem. Iridescence is one that produces colour. This is an interference effect where light is light is diffracted / reflected / refracted from extremely thin layers or tiny regular structures, changing the phase of the light waves so that they can cancel or reinforce different wavelength producing various interference colours. This is similar to the effect seen in ‘oil on water’ or ‘soap bubbles.
The most striking of these is the play of colour found in precious opal. This is due to the orderly arrangement of tiny silica spheres (200nm-300nm in size). This produces a diffraction and interference effect on the light being reflected of the tiny ordered spheres and results in the classic rainbow of colours.
Labradorescence seen in Labradorite and Spectrolite is similar but in this case produced by interference of thin lamellar twining of feldspars. The colours seen are more ‘sheet’ like.
Iridescence can also be produced by flaws and cracks in stones where the cracks have very thin films of air in them allowing interference of light by reflection.

The nature of light falling on a gem can change its colour. Sunlight contains most wavelengths of light. Incandescent and fluorescent lighting have certain wavelengths missing. This has the effect of changing the colour of the gem depending on the light it is viewed in, as certain wavelengths are no longer available to be reflected. Metamerism is the term used but many know it as the ‘ Alexandrite effect’ because the classic example of this effect is shown by the rarest of gems, Alexandrite. This causes the gem to appear green in daylight and red in incandescent light. The more common example is synthetic Corundum doped with vanadium which is pale blue in daylight and amethyst purple in incandescent light. Actually even the time of day can affect colour. Professional diamond graders are well aware of this fact.

Dispersion can also colour gems. The best example of dispersion of light is that if a glass prism breaking up sunlight into its spectral colours. That is also how rainbows are made with dispersion occurring in a myriad of drops reflecting back to you.  When a gem is faceted you have a multitude of small prisms and mirrors each producing a dispersion of rainbow colours.  Generally known as the ‘fire’ of a gem, the degree of dispersion generally depends on the size of the refractive index and can be measured by the difference in refractive index when using the B and G Fraunhofer wavelengths which are near the 2 extremes of human vision. While most striking in colourless gems like Diamond and extreme in Strontium Titanate, coloured gems like Demantoid Garnet and Sphene also exhibit dispersion although masked to a certain degree by the colour of the stone.

This is a slightly abridged version

© 2010 Edward Vabolis

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