Duchess of Cambridge wearing classic pearl teardrop earrings. (Picture: Reuters)

Coco Chanel declared them the mainstay of any elegant woman’s wardrobe, but in recent years pearls have been consigned to granny’s jewellery box, considered too dowdy compared to gems with more sparkle.

Now they’re back in style in a big way. Everyone from the Duchess of Cambridge to Rihanna is wearing them, while designers such as Gucci and Miu Miu are adding them to clothes and accessories — even sticking them to catwalk models’ faces.

Rihanna loving her pearls. (Picture: Instagram)

So, what’s the appeal of a pearl?

Long before diamonds and rubies were mined, lustrous pearls found in oysters were prized. ‘They were the most valuable gem for centuries,’ says David Warren, of auction house Christie’s.

Legend says Queen Cleopatra dissolved a giant pearl in a glass of wine and drank it to show off her wealth to lover Mark Antony.

Historian Suetonius wrote that a Roman general financed a military campaign by selling just one of his mother’s pearl earrings.

And in 1917 a millionaire financier traded a five-story mansion on New York’s Fifth Avenue for a two-strand necklace for his wife.

Unlike gems, which develop underground, pearls are created within living creatures in oceans and freshwater lakes.

Kate is a great lover of pearls. (Picture: AP Photo/Kirsty Wigglesworth, FILE)

A natural pearl starts to form when a foreign object, such as a shell sliver, lodges in an oyster’s soft body. To ease this irritant it secretes ‘nacre’. Layer upon layer then builds up, making a pearl.

It can take five years for a 3mm gem to result. Barely 5 per cent of oysters yield pearls of fine gem quality. A processor has to sift through more than 10,000 to find enough for a 16in necklace.