Blackrock Jewel

The blackrock jewel is a magical by-product of the blackrock dome that functions as a gateway between worlds and appears in Ultima Underworld II.

Description
Due to the crude magic used in the creation of the blackrock dome surrounding Castle Britannia, a series of physical "echoes" of the original construct were created on various planes. The largest of these manifestations was an eight-faceted blackrock jewel, which was created within the lowest depths of the sewers beneath the castle.

Because of the Guardian's reliance on magical energies from other planes, the jewel was able to act as a portal to worlds that had been affected by similar feats of spellcraft. By stepping through the faces of the large gem, the Avatar was able to travel across dimensions to the realms of Tarna, Killorn Keep, Anodunos, Talorus, the Scintillus Academy, the Pits of Carnage, Rhiannon, and the Ethereal Void. Within each of these lands, the hero retrieved another "echo" in the form of a blackrock gem, and by fusing these smaller constructs with the larger jewel, they were able to weaken the structure of the dome itself until such a point that it could be shattered.

Following this eventual sundering of the dome at the sounding of Praecor Loth's horn, the eight-faceted jewel was destroyed, and with it the gateways between Britannia and other worlds.