Many of the grandest pieces of jewelry made their way into Queen Elizabeth II's collection thanks to her jewel-mad grandmother, Queen Mary. Today's earrings, the Gloucester pendants, are no exception, although their history can be traced back generations further than even Mary herself.
The earrings were made at some point during the first half of the nineteenth century. Jewel historians like Sir Hugh Roberts don't pinpoint a particular maker, although it's generally agreed that they were made by an English jeweler. The earrings originally belonged to Princess Mary, Duchess of Gloucester and Edinburgh. She was one of the daughters of King George III, and also his last surviving child. She did not have any children herself, so when she died in 1857, she bequeathed the earrings to her niece, Princess Mary Adelaide of Cambridge.
The earrings that Mary Adelaide received were larger than the ones that the Queen wears today. Along with the pearls pendants surrounded by the elaborate diamond frame, the earrings also featured a detachable stud with a central pearl surrounded by a cluster of diamonds. The earrings also had another setting: the diamond frame could be removed, leaving just the pearl drops suspended from the studs.
Mary Adelaide later became, of course, the Duchess of Teck, and the mother of Queen Mary. Mary's the one who inherited the earrings from her mother in 1897, several years before she became queen. She also apparently preferred to wear just the stud portion of the earrings. In 1947, as her granddaughter, Princess Elizabeth, was leaving for a tour of South Africa, Mary gave her the pearl clusters. Elizabeth wore them later that year on her wedding day.
When Mary died in 1953, the pendant portion of the earrings were also left to Elizabeth. By that point, a separate diamond stud had been attached to the earrings. The Queen has made them an important part of her jewel rotation, often wearing the pendants with two other diamond and pearl pieces: the Vladimir Tiara and Queen Victoria's Golden Jubilee Necklace.
The earrings are made of typical nineteenth century materials: gold and silver along with the diamonds and pearls. Because of this, I'd expect them to be significantly heavier than some of the Queen's newer pendant earrings (like the platinum-set Greville chandelier earrings, for example). I'm guessing that's one of the reasons that HM tends to select other earrings for white-tie occasions these days, leaving the Gloucester pendants in the back of the jewelry box for now.