L Croissant Purple Sapphire Ring

Sapphire is corundum: crystallised aluminium oxide, 9 on the Mohs scale, harder than any gem but diamond. Pure corundum has no colour at all; every colour comes from a trace element. Iron and titanium make it blue, chromium makes it pink, and the two together purple. When chromium turns it fully red it is called ruby instead, so every other colour of corundum, orange and yellow included, is sapphire.

The name comes through Latin from the Greek sappheiros, which probably meant lapis lazuli. Medieval Europe gave blue sapphire to kings and bishops as the colour of heaven, and a sapphire said to have been Edward the Confessor's still sits in the cross at the top of the British Imperial State Crown. Today almost every sapphire is heated to improve its colour, which makes one left as it came from the ground the rarer thing.

Sapphire is found in many parts of the world: Sri Lanka, worked for more than two thousand years, Madagascar, Myanmar, Thailand, Tanzania and Australia among them. Kashmir produced the most coveted blues of all, from a small deposit found in 1881 that gave its best within a few years.

Madagascar became one of the world's great sapphire sources almost overnight, when a find at Ilakaka in the south-west in 1998 started a rush that built a town of tens of thousands within a few years. Madagascar and Sri Lanka were neighbours in the ancient supercontinent of Gondwana, and their sapphires share that geology. That is why Madagascar, too, gives sapphire in every colour, and is known especially for its pinks and purples.

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Take it off for the gym, the sea and the garden. Warm water, a drop of soap and a soft brush; dry with a soft cloth.