A rich heiress in The Merchant of Venice, in love with
Bassanio. Her father had ordained that three caskets should be offered
to all who sought her hand- one of gold, one of silver, and one of
lead- with this proviso: he only who selected the casket which
contained the portrait of the lady should possess her hand and fortune.
(Shakespeare.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894