There is a common misconception that all sapphires are blue and all rubies are red. However, there’s a lot more to the colour of these stones than one might think.
Both of these precious gemstones belong to the same family of stones, called corundum. Scientifically speaking, all corundum (both sapphire and ruby) is created equally in terms of its chemical composition. That is to say, it is all made from the chemical compound, aluminium oxide.
So how is it that they are different colours?
Corundum is what is called an allochromatic gemstone, which means that its colour comes from small amounts of impurities within the crystal. A piece of corundum without any impurities present is colourless (i.e. a white sapphire). However, it is when some other elements are added that we see the amazing range of colours that a sapphire can possess.
Most of these foreign elements are some type of metal. For example, if there is some iron present then the sapphire can take on a yellow or green colour. Some titanium alongside the iron will produce the beautiful shades of blue for which sapphire is so famous.
An orange or pink sapphire is formed when some chromium (rather than titanium) is mixed in with the iron and, if there is sufficient chromium, the gemstone becomes a ruby (which is simply the name given to what is essentially a red sapphire).
The debate between gemologists and gem dealers about when a pink sapphire becomes a ruby can sometimes get quite animated, especially since being classified as a ruby typically increases the price of the gemstone quite significantly.
Interestingly, there are certain countries that prefer particular colours of sapphire, so the market prices can change depending on where you are buying and selling.
However, regardless of the colour, the other properties of corundum (such as its resistance to scratching and incredible lustre) remain the same, meaning sapphire and ruby continue to be some of the most popular gemstones in the world.