Her simple, midnight blue Jenny Packham gown was the epitome of elegance – but the real centrepiece of her outfit – and the addition that got everyone talking – was the exquisite diamond necklace that hung from her neck.
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The stunning necklace which the Duchess wore was on loan from the Queen. It was a wedding present from the Nizam of Hyderabad – the monarch of India's Hyderabad State.
The Nizam asked the Queen – then Princess Elizabeth – to choose two pieces of jewellery from Cartier to celebrate her 1947 wedding. She chose a tiara, which has now been taken apart and make into other pieces, and a necklace based on an English rose. It is still worn on occasion by the Queen today.
Cartier originally crafted the necklace in the 1930s. Originally it consisted of a long chain of 46 diamonds – which was eventually reduced to 36 – and a diamond-encrusted snap.
Its breathtaking centre-piece is pave-set with a detachable double-drop pendant, 13 emerald-cut diamonds and a pear-shaped drop.
The original version – which was purchased in 1936 but bought back by Cartier a year later – had three triple-drop and eight double-drop pendants.
The Queen wearing the Nizam of Hyderabad necklace
Nine of these - eight double-drop and the larger of the triple-drop pendants - were removed and sold, however.
The glittering necklace perfectly complimented the dress code for Kate's evening out, which was "black tie with a hint of sparkle".
Prince William's wife – who was at the National Portrait gallery in her role as patron – met VIP guests including actor Alan Rickman, photographer David Bailey and transvestite potter Grayson Perry before attending a gala dinner.