When you can see two colours emitted from different angles of a gemstone the effect is caused by dichroism.
As light enters a transparent or translucent gemstone it often splits into two distinct rays. In fact of all the well-known gemstones, all except Diamond, Sphalerite, Spinel and Garnet, are dichroic.
Some gems are more dichroic than others. If as you turn a gem around you see different colours from different angles then the gemstone is a dichroic gemstone. If you can see two colours the gem is known as a dichroic gemstone and if three colours are seen then it is said to be pleochroic. If you can see more than three colours, then one or more are being caused by another optical effect, as only a maximum of three colours will be witnessed through pleochroism.
With Tanzanite, Iolite and Tourmaline (Tourmalines are the most dichroic of all gemstones) the angle of the split is so large that different colours can be seen with the naked eye, however in other gems such as Sapphire and Ruby the angle of the split is more acute and the naked eye cannot normally detect the differences in colour.
In Zircon the split is so pronounced that if you placed a large piece over a newspaper and tried to read it, you would think you had developed double vision as you would see two lots of text! This is also known as double refraction.
So when you see the colour of the rainbow in a flash of sparkle from within a Diamond, it’s important to realise that this is not the result of diochroism (light being split in two), but from the division of a single beam of light being split into its constituent components. Interestingly and quite confusingly, those gemstones that disperse light the most (Sphalerite, Demantoid Garnet and Diamond), do not split light when it enters a gemstone and are not dichroic.
