This brooch features 18 large diamonds around a smaller, detailed gold setting of diamonds, which encircles a large central sapphire. The piece compliments blue and purple outfits well.
Queen Mary bought the brooch in the 1930s along with a number of other pieces, but the history is complicated. From Her Majesty’s Jewel Vault explains this:
Queen Mary acquired the brooch in 1934, and it is said to have been one of the jewels she purchased from the daughters of the late Empress Marie Feodorovna, who sold some of her jewels after her death; there was companion brooch from that Russian collection as well. Other sapphires were purchased by her at that time as well, including the sapphire and pearl brooch The Queen wears today, though it is worth noting that the records that exist from the sale of those jewels list Mary’s last purchase in 1930.
It was actually first worn by Her Majesty in 2014, though was passed from Queen Mary to Elizabeth upon her accession in 1952. The Queen Mother regularly wore the brooch, so it seems her daughter loaned it to her.