The chests that Portia's suitors must open, like the rings that she and Nerissa give their husbands to safeguard, none-too-subtly evoke the female genitalia. In the final scene, when Portia and Nerissa pretend to have slept with the lawyer and the law clerk to whom their rings were given, they make this connection explicit. By using precious objectsand, in the case of the stones and the rings, objects of commercial exchangeto stand for human sex, Shakespeare links the supposedly pure spheres of love and marriage to the play's exploration of money and greed.
When Shylock raves about the "stones" that Jessica has stolen from him, part of the joke is that in the Renaissance "stones" was a slang word for the testicles. And indeed Shylock's only child's renouncing her father, eloping, and converting to Christianity is symbolically tantamount to castrating him, cutting off his family name. Multiple characters undergo kinds of symbolic castration throughout the play. Antonio, who seems not to expect to marry or have children, refers to himself as a "wether," or neutered ram. Portia's suitors, who vow never to seek other wives, also forfeit their ability to produce heirs.
The The Merchant of Venice quotes below all refer to the symbol of Stones, Rings, and Caskets. For each quote, you can also see the other characters and themes related to it (each theme is indicated by its own dot and icon, like this one: Note: all page numbers and citation info for the quotes below refer to the Simon & Schuster edition of The Merchant of Venice published in 2009.).