You will often hear jewellers talking about the ‘fire’ of a gemstone, especially in Diamonds.
Fire, or dispersion, is caused when a gemstone splits white light into the seven main colours visible to the naked eye by refraction. The light disperses in the gem and reflects on its inner surfaces.
Refraction in a gemstone is caused by the change in speed as light travels from air into the gemstone. The specific properties of the gemstone slow the white light down until the different wavelengths separate into the seven colours: red, orange, yellow, green, blue, indigo and violet.
However, the human eye can only see three colours; red, blue and green. These are called additive primaries, and it’s from a mixture of these three colours that the human brain can create every possible colour. It’s very easy to concentrate on the physics of this phenomenon without looking at the sheer beauty and rarity. Nothing catches your attention more than a flash of colour from a Zircon or a Diamond.
Having mentioned these two gemstones, it is worth pointing out that Diamonds do not actually display the best fire possible in a gemstone. Demantoid Garnet and Sphene have a greater fire than Diamond, and Sphalerite’s dispersion is three times more!
