I think it was costume jewellery - quite unusual for Queen Elizabeth II, even more in combination with genuine diamonds (earrings, necklace, bracelet). We can see on this picture that the brooch's "diamonds" do not sparkle like real diamonds, they are rather probably mere Swarowski crystals, strass or rhinestones.

If I remember correctly, Angela Kelly had written in one of her books that she occasionally (though not frequently) used costume jewellery to enhance the Queen's outfit. This was probably one of these occasions she refered to.