This is a Van Cleef & Arpels brooch, owned by Queen Camilla.
In 1967, the ballet choreographer George Balanchine created an original ballet called Jewels and Van Cleef & Arpels made a series of broochs to celebrate the ballet.
In 2007, to celebrate the 40th anniversary of the ballet, the Royal London Ballet performed Jewels in memory of Balanchine , and Van Cleef & Arpels reissued copies of the original brooches, as well as some new ballerina designs.
Queen Camilla owns the one pictured above which is made of diamonds, rubies and sapphires set in gold. Here she is wearing it.