Queen Catherine de Medici (1519-1589), who married KingHenri II (1519-1559), bought the château in 1550. The Domaine was highly profitable at this time (toll on the Loire and countless farming plots). She probably used Chaumont-sur-Loire as a hunting ground and stop-over between the Châteaux d’Amboise and Blois.
Catherine de Medici surrounded herself with astrologers her whole life, the most famous being Nostradamus (1503-1566) and Cosimo Ruggieri (?-1615). Legend has it that it was at Chaumont that Ruggieri predicted the end of the Valois dynasty to Catherine de Medici, in favour of the Bourbon dynasty with the arrival of Henri IV (1553-1610), King of Navarre. Cosimo Ruggieri made the faces of the Queen’s three sons destined to reign appear in a mirror. The number of times the mirror rotated indicated the number of years that each of the three kings would reign: François II (1559 - 1560), Charles IX (1560 - 1574) and Henri III (1575-1589).
Upon Henri II’s death, during a tournament, in 1559, Catherine de Medici –who had become Ruler of France– asked her former rival Diane de Poitiers to give her back the Château de Chenonceau. Indeed, this gift from the king was an inalienable property since it belonged to the Crown. In exchange she gave Diane de Poitiers (1499-1566) the Château de Chaumont.
Henri II’s former mistress only stayed at Chaumont on rare occasions, but, concerned about her residences, she continued with the château’s construction up until her death in 1566. Chaumont now owes most of its current appearance to her. Upon becoming the new owner, her daughter was the first to express a desire to endow the château with grounds in 1573. But her death the following year put an end to the plans.