One of our favourite gemstone quirks is a natural phenomenon called colour zoning. Colour zoning is where you can see bands of different colours inside a gemstone and is common in species of gem that are coloured by impurities, such as sapphire.
Colour zoning occurs during the growth of the crystal and is a result of a changing environment as the crystal grows. As different trace elements that colour the crystal become prominent the colour of the crystal shifts, and this creates bands of various colours. These bands follow the crystal structure of the gemstone which can result in some beautiful patterns through the final fashioned stone, much like the hexagonal zoning patterns that can be seen in the deep green sapphire below. Interestingly this sapphire looks green to the eye but it is in fact deep bands of blue and yellow that create the body colour, typical of sapphires mined in either Australia or Thailand.
Colour zoning can be subtle and only observed with a loupe, or it can be incredibly prominent and eye visible. Traditionally colour zoning was not considered a desirable trait in gemstones, but today some lapidarists will specifically fashion gems to accentuate the beautiful story of the stone and we love the result. Some more obvious examples of colour zoning can be found in ametrine (half amethyst and half citrine) as well as in parti coloured tourmalines.
As a gemmologist colour zoning is incredibly useful in diagnosing whether a gemstone is natural or synthetic. For example, the hexagonal crystal structure of the sapphire means that only straight colour zoning will be present, whereas the growing process of synthetic sapphires means they tend to exhibit curved zoning. A beautiful insight into natural crystal growth we love seeing colour zoning in our pieces.