Moonstone Opal

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 Sun-Drop Flower.

Its destructive powers cuase death, ruin, decay, disintegration, nothingness and destruction to those around it, enough to render an entire kingdom uninhabitable and devastated. As with the mystical yellow Sundrop Flower, its powers can be activated only by singing the decaying incantation. As such it's the embodiment of death-force itself.

Since forming the unnamed dark kingdom to further horde and exploits its evil powers to those around it, the Opal resided in a special chamber, waiting to reunite with Sun-Drop Flower's new form.

In an unexpected twist, the Opal has allowed itself to merge with Cassandra's entire body; resting on the upper center of her chest, glowing a light shade of moonlight blue.

Trivia

 * Since it had 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. It is now one with the former loyal, once devoted handmaiden; being embedded on the right side if her chest.
 * In Tinker Bell and the Lost Treasure, the anciently powerful Moonstone is a dark blue glassy orb, from which light blue fairy dust is created during the eight-year Autumn Revelry. Only one exists after three centuries.