Does anyone know the provenance of Catherine's necklace? I know Camilla's worn it at least once, but is it part of her personal jewelry collection or a long-unseen piece from the jewel vault? It's a gorgeous piece.

The Princess of Wales is wearing the Queen Mother's Amethyst Necklace. It was given aa a wedding gift from Queen Alexandra to the then future Queen Mother in 1923. As you say it's been publicly worn by Queen Camilla.