The Sapphire Tassel Demi-Parure (Necklace, Earrings, Bracelet)
This set of sapphires and diamonds is centered around a necklace of multiple diamond strands which come together in a central knot accented by a sapphire, with a tassel of three diamond strands with sapphire pendants hanging below. It also includes earrings, each with a cluster of three sapphires surrounded by diamonds. The Queen tends to wear a bracelet with multiple sapphires separated by diamonds with diamond straps with the set which may be a part as well, though no official information has been released so it is hard to tell what is technically a part of the set. She wears sapphire rings with all of the above, though I've never seen them well enough to know if any belong specifically to this set.
Though no information on the set's provenance has been released, we can make an educated guess: this was worn to the state banquet during the 2007 state visit from King Abdullah of Saudi Arabia, which is a strong indication that it is a Saudi Arabian gift. Some speculated at the time that it could have been a gift from King Abdullah when he was the crown prince, though when the gift was received is unknown (it's been worn publicly since at least 2002).