Jessica, Shylock's only child, elopes in Act 2 of The Merchant of Venice. While most would think that her father would be concerned about or angered by her absence and her marriage to a Christian, Shylock is instead furious that Jessica took his jewels with her.  He sends his friend and fellow Jew Tubal to try to find Jessica as she and her new husband Lorenzo flit around Europe, but Tubal is always one step behind.  In Act 3, Scene 1, when Tubal returns to Venice, he tells Shylock of the numerous rumors that he heard about Jessica and Lorenzo spending money and says,

" 'One of them showed me a ring that he had / of your daughter for a monkey' " (3.1.116-117).

Shylock becomes even more enraged by this news because the ring is a turquoise one given to him by his deceased wife. Additionally, monkeys during Shakespeare's time were symbolically lustful; so in Shylock's eyes, his only child has taken a sentimental and precious item and traded it for a cheap, pagan animal.