Her Majesty was meeting with Agnes Oswaha, Ambassador from the Republic of South Sudan, on Tuesday.
She can be seen wearing a floral dress with the Onslow Butterfly Brooch, which has only been seen in public three times in the last decade.
The brooch is in the shape of a butterfly and features diamonds and rubies.
Last November, Her Majesty celebrated her wedding anniversary by wearing her Chrysanthemum Brooch.
The sapphire and diamond brooch was a piece she wore for a series of portraits taken with Philip on their honeymoon in 1947.
On Wednesday, the Queen accepted a special rose from the Royal Horticultural Society in honour of her husband’s 100th birthday.
The rose was planted in the gardens at Windsor Castle in his memory.
Her Majesty said the flower “looks lovely” and it was a “very kind” tribute.