Which figure of speech is used in this excerpt from act l of Shakespeare's Romeo and Juliet

The figure of speech used is simile.

Juliet is being compared to a rich jewel against an Ethiopian’s ear. Similes are often preceded by words such as "like," when comparing two objects to make the description more vivid. The simile used depicts that Juliet is outstanding that she is as distinct as a jewelry against the dark skin of an Ethiopian.