There's nothing unusual about seeing the Queen wearing pearls. Indeed, she wears pearl and diamond earrings and a three-stranded pearl necklace at nearly every daytime engagement she attends. But one of the lesser-seen necklaces in her collection is the four-stranded pearl choker necklace with an unusual diamond clasp.
Leslie Field writes that the necklace was made "from pearls in [the Queen's] collection." The pearls were reportedly a gift from the Japanese government, possibly acquired during the 1970s, when she made a state visit to that nation. The necklace was constructed by Garrard. Field also speculates that the clasp of the necklace is large enough to accommodate an additional strand of pearls, should the Queen ever choose to make this a five-stranded choker.
In November 1982, the Queen famously loaned this choker to her new daughter-in-law, the Princess of Wales, to wear during a state visit from Queen Beatrix of the Netherlands. Diana donned the choker for a banquet at Hampton Court Palace, wearing it with Queen Mary's Lover's Knot Tiara and the enormous sapphire brooch given to her by the Queen Mother as a wedding present. (It's also one of the few times Diana wore the sash of a chivalric order; she's wearing the Order of the Crown, given to her by Queen Beatrix during this particular state visit.)
The Queen has also worn the choker for a tiara affairs, pairing it with Queen Alexandra's Kokoshnik or the Girls of Great Britain and Ireland Tiara, as she did in Bangladesh in 1983.