Sapphires belong to the corundum family of gemstones, along with rubies. What makes them so desirable, apart from their beautiful colours, is that corundum gemstones have a hardness on the Mohs scale of 9 which means they are very hard and suitable for use in jewellery such as rings which may get some wear and knocks. The difference between sapphires and rubies is determined by colour as they are both composed of aluminium oxide. The addition of chromium to the aluminium oxide results in a ruby while the addition of iron and titanium to the aluminium oxide results in a blue sapphire.
Sapphires come in many colours although the best known colour is blue. When referring to sapphires that are not blue, the correct description is a pink sapphire or a yellow sapphire, but it is understood that when a stone is just described as being a sapphire, it is a blue stone.
The main colours of the sapphire family are blue, yellow, pink, colourless (or white), and black. There can also be orange, green and purple. One extremely rare colour sapphire is the padparadscha, which is a vibrant mixture of pink and orange. There is also a change colour sapphire. A rare stone is a star sapphire, showing asterism or rays like a star.
It is the addition of chromium that will lead to a pink sapphire (ultimately to a ruby if there is enough chromium), iron leads to yellow and green, and vanadium leads to an orange sapphire.
Colourless (or white) sapphires were often used in antique jewellery as an imitant of diamonds and are an overlooked gemstone.