Have you ever wondered what makes gemstones the colour they are? There are two different ways they gain their colour – idiochromatic gems are those which contain a colour-inducing element as part of their chemical composition. Peridot and Malachite are examples of this. Most gems, however, are found as colourless gems and impurities in the gem, or crystal lattice deformations cause their various colours. These are called allochromatic, which describes any gem you can think of which is available in more than one colour. A short list of elements is responsible for these variations in colour. Perhaps the most important is the element chromium.
Named after the Greek word ‘chroma’, meaning simply ‘colour’, chromium can create a whole host of bright colours in various compounds. Chromium has the chemical symbol Cr and sits at 24 on the periodic table (meaning that it has 24 protons in its nucleus). It was first recognised as an element in 1798, when it was isolated by Louis Nicolas Vauquelin, who even noted its presence in Ruby and Emerald. Known for its anti-tarnishing properties, chromium is used in stainless steel and chrome plating. This property was known in the ancient world, where it was sometimes used to tip weaponry. Chromium is a transition metal, which means that its electron shell isn’t completely filled. This means that it forms in many states and can form numerous compounds. It is usually found in nature in compounds, but is very rarely found in pure form in kimberlite pipes alongside Diamonds. Chromium III is found as an interstitial in many gemstones – meaning that it is an impurity in the crystal structure, or as a substitution, where it replaces another atom.
One of the things that is so fascinating about chromium is that it doesn’t cause just one colour in gemstones. It usually causes gems to be green, but it can also cause red, blue, or even colour change in certain gems. It makes both Topaz and Ruby red, but how? Let’s use Ruby as an example, which has a chemical formula of Al2O3. The aluminium is in the form Al3+, which happens to be a similar size to Cr3+, which replaces around one in a hundred aluminium atoms, becoming surrounded by six O2- ions. However, because Cr3+ has an incomplete electron shell, the unpaired electrons can get excited and can absorb particular wavelengths of light, giving a gem its colour. In the case of Ruby, green and violet wavelengths are absorbed and red light is transmitted, giving the gem a red colour.
Interestingly, the chromium in Ruby also causes a very brief fluorescence (the ability to glow when exposed to light) of a few nanoseconds in the red wavelength, which adds to the vividness of the gem’s colour and lustre. So chromium actually creates the red colour in a Ruby in two different ways concurrently – by transmitting and simultaneously fluorescing red light.
More usually, chromium impurities cause a gem to be seen as green. Green gemstones coloured by chromium include Emerald, Chrome Diopside, Chrome Tourmaline, Jadeite, Tsavorite Garnet and Demantoid Garnet. So why does the same impurity cause these gems to become green? The answer is in the way chromium interacts with the host molecules. In the case of an Emerald, the bonds between the Beryl and the chromium are slightly weaker than in a Ruby. These weaker bonds produce a huge shift in the bandwidth of light absorbed by the gem. Yellow-red is absorbed by the gem and blue-green is transmitted, producing a green colour to the eye.
Chromium causes some gems to turn blue, such as some Topaz, Aquaprase™ and (along with iron and titanium impurities) Kyanite. This occurs in the same way, but with even weaker bonds between atoms, the bandwidth of light absorbed shifts again to produce a blue colour. Even more astonishingly, chromium causes almost all colour change in gemstones. The only gem which can change colour which is not coloured by chromium is Colour Change Fluorite. Colour changing varieties of the following gems are all coloured by chromium – Alexandrite, Csarite®, Garnet and Sapphire, as well as extremely rare museum pieces of Spinel and Kyanite. In these instances, the chromium causes two transmissions in equal balance – blue/green and red. Because these colours are in balance, they appear different colours in different lighting conditions. Daylight contains more blue light, so more blue light is returned to the eye. In incandescent light, which has more long-wave red light, red is returned to the eye. This means that the same gem can appear blue to green during the day and red under candlelight. All colour change gemstones work in this same way.
The change in interaction between chromium and its host mineral cause it to create a different colour in different gemstones. A concentration of chromium as low as 0.03% can create a spectacular colour, or even a colour changing gem. Unfortunately, chromium makes up only 100 parts per million of the Earth’s crust and it is not evenly distributed across the planet. Finding places where it has found its way into the crystal lattice of a gemstone which is already rare, sadly makes these spectacularly coloured gems extremely difficult to find.
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