Given to Princess Elizabeth before she became Queen, the necklace is the most valuable piece of jewellery owned by any Royal Family. It is worth millions, according to new research.

But the Nizam didn’t stop at the necklace: he gave Elizabeth permission to choose another piece from his impressive jewellery collection.

She opted for a floral tiara, which featured removable silver flowers that have since been worn by the Queen as brooches.

The necklace matches the tiara and both pieces are covered in diamonds and set in platinum.

Created by Cartier in 1935, the necklace is intricate and features geometric diamonds which are nestled in the floral design.

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Hugh Roberts, who was the Director of the Royal Collection and Surveyor of the Queen’s Works of Art from 1996 to 2010, once descried the jewel.

He said: “The pavé-set centre with detachable double-drop pendant incorporating 13 emerald-cut diamonds and a pear-shaped drop.

“The chain of 38 brilliant-cut open-back collets with an elongated oval brilliant-set snap.”

Elizabeth wore the necklace on multiple occasions after her marriage to Prince Philip, and even wore it for a series of portraits taken before her coronation.

Kate donned the vintage necklace with a black velvet dress designed by British favourite Alexander McQueen.

As for other jewellery, she wore dangling diamond earrings and the Princess Diana Lover’s Knot tiara.

The diamond and pearl headpiece was given to the late Princess as a wedding gift from the Queen in 1981.

Kate first wore the tiara for a previous Diplomatic Corps Reception, in 2015.

The world’s top 10 most expensive royal jewellery pieces

Nizam of Hyderabad necklace – £66.3million

Cullinan III and IV brooch – £50million

Williamson Diamond brooch – £25million

Greville Emerald Kokoshnik tiara – £10million

Queen Maud’s diamond tiara – £10million

Henckel von Donnersmarck tiara – £9.2million

Swan Lake necklace – £8.7million

Noor-Ul-Ain tiara – £8.7million

La Peregrina pearl – £8.6million

Greville Festoon necklace – £8.2million