The Moonstone Opal is a supremely powerful, ancient opal gem that had descended to Earth from a single drop of pure moonlight thousands of years ago, along with its heavenly counterpart of healing and light - the golden Sundrop Flower.
In contrast to the Sundrop, which is an object of healing and restoration, the Moonstone Opal has destructive powers that cause death, ruin, and decay to those around it, enough to render an entire kingdom uninhabitable and devastated. However, its powers can be activated only by singing the Decay Incantation. As such, it is the embodiment of death-force itself. But, the Moonstone Opal's power can potentially be used for good, if wielded in tandem with the power of the Sundrop Flower by someone whose intentions are good (such as Rapunzel).
Since forming the Dark Kingdom with it's own power, the opal resided in a special chamber, waiting to reunite with the Sundrop Flower's new form. It was responsible for the creation of Black Rocks, large rocks that erupted throughout multiple lands, particularly in the kingdom of Corona and the surrounding area, as the Moonstone Opal sensed that the Sundrop Flower's new form resided there. As warned by Lord Demanitus, failure to reunite the Moonstone Opal with the Sundrop would result in more black rocks being created indefinitely until everything was destroyed by them.
In an unexpected twist, Cassandra snatches the Moonstone and merges with it, resting on the upper center of her chest, glowing a light shade of moonlight blue. If Cassandra wishes to use its power, however, she needs to tap into her rage in order to awaken the opal's power and connection to the black rocks it creates.
- Since the Moonstone Opal descended onto Earth, it had never been coveted by any mortal until centuries later by none other than a vengeful, power-mad Cassandra. As such, it resided safely within an ancient chamber designed to withstand its immense heavenly powers of death and darkness.
- The Moonstone Opal's title, blue color and its unique connection with the moon, is a bit similar to the fragile Moonstone and its shattered shards from Tinker Bell and the Lost Treasure.
- According to concept art by Brigette Barrager, the Moonstone was originally intended to possess Cassandra. Although it was scrapped mid-production, Cassandra's skin evidently appears pale after merging with the Moonstone, implying that it may have a physical effect on her.