According to Leslie Field in The Queen's Jewels, pg 168:
At the apex, below the the diamond-set globe, hang four large pear-shaped pearl drops. The drops were probably among a collection of pearls presented to Catherine de Medici by her uncle, Pope Clement VII. She gave them to Mary, Queen of Scots, who married Queen Catherine's son, the Dauphin of France. After Queen Mary's execution, her magnificient collection of pearls was bought by Queen Elizabeth I for 4,000 pounds. These became part of Queen Victoria's inheritance and the four pearls have been in the crown since 1838.