In the 'Poirot' episode "The Double Clue", a priceless necklace was stolen from the safe of Marcus Hardman, which he claimed once belonged to Catherine de Medici.
According to Wikipedia, Catherine de' Medici (April 13, 1519 – January 5, 1589) was born in Florence, Italy, as Caterina Maria Romola di Lorenzo de' Medici. Her parents, Lorenzo II de' Medici, Duke of Urbino, and Madeleine de la Tour d'Auvergne, countess of Boulogne, both died shortly after she was born. Under the Gallicised version of her name, Catherine de Médicis, she was queen consort of King Henry II of France from 1547 to 1559.
Catherine de Medici was portrayed in Toobworld several times. For 'Elizabeth R', the role was played by Margaretta Scott in 1971; Pamela Brown assayed the part for an episode of 'Sir Francis Drake': "Mission To Paris", and in 'The Conquest Of America', it was played by Lisa Wolfinger.
The picture here is of Joan Young as Catherine de Medici in two episodes of 'Doctor Who' before his first regeneration.