Camilla wore the sapphire collection, sometimes known as the King George VI Victorian Sapphire Suite.
It is said to have been of great sentimental importance to the late Queen.
The necklace was a gift from her beloved father, George VI, for her wedding to the Duke of Edinburgh in 1947.
Camilla was also dressed in a royal blue lace evening dress by Bruce Oldfield, the Garter sash, Queen Victoria's garter order and the late Queen's family order.
Camilla joined King Charles as he welcomed South Africa's President Cyril Ramaphosa earlier today in another milestone for the new monarch.
The pair joined more than 160 guests for a glittering white-tie evening affair at Buckingham Palace in honour of the South African president.
Prince William and Kate Middleton also arrived wearing their evening best.
Kate wore a caped, white Jenny Packham floor-length evening dress with sparkling detail on the shoulders, and her go-to tiara, the Lover's Knot.
Kate also wore the family order, and the GCVO Star and Sash, as well as the four-strand pearl bracelet belonging to the late Queen and Diana's diamond and pearl drop earrings.
The Lover's Knot Tiara, a diamond and pearl-encrusted headpiece made in 1914, was often worn by her namesake Diana, Princess of Wales.
It was given to Diana by the Queen as a wedding gift in 1981.
King Charles is wearing the Garter Star and Thistle Star, along with three miniature medals (for the Gold, Diamond and Platinum Jubilee).
Other members of the royal family that have arrived include the Earl and Countess of Wessex, the Duke and Duchess of Gloucester and the Duke of Kent.
His Majesty will be treating Mr Ramaphosa to a luxury menu of stuffed Windsor pheasant and grilled brill with wild mushrooms tonight.
The lavish menu offers two mains of grilled brill and Windsor Pheasant.
Some of the sides included a selection of Chantenay carrots, kale with roasted butternut squash, braised fondant potatoes and salad.
And for dessert, guests will enjoy an iced vanilla parfait with caramelised apples or coffee and bite-sized confectionery were the options for dessert.
Some of the wines on offer include £75 Taylor's Vintage Port 1983, a Château Feytit-Clinet and a Pomerol 2000.
Other bottles of wine include a £60 Ridgeview Blanc de Blancs 2016, an English sparkling wine that goes at £60 a bottle, a £77 Chassagne-Montrachet 1er Cru, a Morgeot, Clos de la Chapelle, a £61 Domaine Duc de Magenta 2014, and a £55 Château Rieussec 1er Grand Cru Classé Sauternes 2007.
Earlier this evening, the King and Queen Consort followed the late Queen's tradition of inspecting the table to make sure everything was in order, ahead of the evening event in honour of the South African president Cyril Ramaphosa.
Tonight, the King personally requested that only sustainable flowers be used on for massive dinner table tonight.
In the Buckingham Palace ballroom, the grand horseshoe-shaped table, set for 163 people, was decorated with blooms sourced from the palace gardens and Windsor Castle.
And in a new addition to the traditional floral centrepieces at banquets during the late Queen's reign, the table was also lined with hundreds of tiny individual stem glass vases, each filled with pink, red and purple flowers.
Cyclamen, nerines, rosehips, anemones, amaryllis, chrysanthemum blooms and hydrangea made up the elaborate display, as well as crab apples sourced from Kent.
The foliage included variegated berried ivy, trailing green ivy, flowering viburnum, mahonia japonica and berried cotoneaster.
A spokesman for the King said: "It was the King's decision to ask for sustainable flowers.
"They are all seasonal and from the gardens of Windsor and Buckingham Palace. They have not been flown in from around the world."
Other high-profile guests tonight will include Prime Minister Rishi Sunak, Lord Speaker John McFall, Baron McFall of Alcluith, and Baroness Amos.