Opal is the fusion of Pearl and Amethyst. She made her debut in the episode "Giant Woman".
Role in the series
Amethyst and Pearl are extremely powerful when they are willing to work together in the form of Opal. Amethyst's ability to live in the moment along with Pearl's obsessive single-mindedness results in a stable fusion. Opal achieves a peace in that balance that Amethyst and Pearl rarely experience on their own. At times, Opal is so calm and at peace that it causes her to forget the task at hand (for example, forgetting to retrieve the Heaven Beetle after rescuing Steven). In addition, according to the book "Guide To The Crystal Gems", Opal is very forgetful.
The balance that holds Opal together is, by far, the thinnest of all the other known fusions. The ease with which Opal can separate is very different from that of Garnet, who is almost impossible to unfuse except under extreme emotion, or if by force (Ruby and Sapphire can willingly separate anytime they wish). If Pearl and Amethyst have conflicting views while fused, this can cause Opal to revert to her constituent Gems; the slightest dissonance between the two poses a danger of involuntary defusion.
Opal's personality is theorized to be a combination of Amethyst's power, along with Pearl's grace and decisive determination. She appears to be more open to Steven's desire to have fun and play, as she sang part of his song back to him. This is a trait that can be traced back to Amethyst.
In "Fusion", Opal shows both of Pearl and Amethyst's personalities by posing with elegant and silly poses.