For many people, the word “sapphire” is synonymous with “blue”, but sapphires actually come in a multitude of other enchanting colours. This may come as a surprise to those who are only familiar with the azure variety of this gemstone, and some may wonder where these other colours of sapphire come from. To understand the various colours of sapphire, one must first understand where blue sapphires come from.
In fact, the word “sapphire” is not the name of the mineral from which these alluring gems are cut, but is the name for the gemstone variety of corundum: the mineral form of aluminum dioxide (Al2O3). Like many minerals, corundum is white or colourless in its pure form, and slight structural impurities produce the colour centers responsible for the gorgeous hues seen in sapphires. In the case of blue sapphires, these impurities consist of ferrous iron (Fe2+) and titanium (Ti4+), which interact with one another to form sapphire’s famous blue hue (specifically, the blue results from a charge transfer, producing Fe3+ and Ti3+).
A white sapphire. Image: Skyjems
Beyond this, there are a number of other elements which may be found as impurities within sapphire that are behind the polychromatic nature of this gemstone. Impurities of ferric iron (Fe3+) may produce a yellow colour, and when mixed with ferrous iron (Fe2+) may also produce a green colour. Small impurities of chromium (Cr3+) can cause a pink colour. A mixture of chromium (Cr3+) with ferrous iron (Fe2+) and/or titanium (Ti4+) can cause a purple colour, but some shades of violet may also be caused by vanadium (V3+). A mixture of chromium (Cr3+) and ferric iron (Fe3+) can produce an orange colour, but in some cases vanadium (V3+) may also play a role. As mentioned previously, corundum gemstones may be colourless if no significant impurities are present; this is known as “white sapphire”.
A yellow sapphire. Image: Skyjems
A pinkish purple sapphire. Image: Skyjems
Unusual mixtures of impurities can also produce commonly recognizable colours in sapphires, or may even change hues depending on the lighting conditions under which the gemstone is viewed. This is the case with colour-change sapphires (in which Cr3+ and/or V3+ is typically present); the most valuable colour of sapphire other than blue is a distinct orangish-pink colour known as “padparadscha”, loosely meaning “lotus blossom” in Sinhalese, resulting from a uniquely proportioned mixture of chromium (Cr3+) and ferric iron (Fe3+).
It is also possible for the impurities present in a sapphire to differ across multiple colour zones producing bi-colour, tri-colour, and in some cases varicoloured gemstones; when there is a clear contrast between these varying colour zones upon gazing into the gem, this is known as “parti coloured” sapphire, a term which derives from the French word “partir” meaning “to part or separate”.
Sapphires may also be so heavily included that they appear to be black, a colour not often seen in faceted sapphires but more commonly encountered in star sapphires, where the dark body colour of the stone provides exceptional contrast behind the gem’s asterism.
Sapphires essentially come in every colour of the rainbow save for a single exception: red. Corundum does exhibit a red colour when the chromium (Cr3+) content is high enough, but this variety of corundum possesses its own name: ruby. Ruby is the only stone separated from the rest, while all other colours of corundum are known as sapphire. This distinction has its roots in the practices that were used to identify gemstones in the ancient world, where colour was originally one of the most important distinguishing factors. Similarly, emerald and aquamarine were thought of as two distinct stones in the ancient world based upon differences in colour, but it is now known that they are merely two varieties of the same mineral which we now call “beryl”.
With so many variations in colour, there is truly no limit to the beauty of sapphires and their versatility in jewellery. If selecting a gemstone as a keepsake, let the variegated character of sapphire provide ample options from which to choose or design that special treasure.