Blackrock prism

Small prisms frequently occur as a byproduct of large scale magical castings involving the mineral blackrock. Such prisms feature prominently in  Ultima VII and in  Ultima Underworld II.

History
The massive blackrock constructs created by the Guardian contained miniature "echoes" of their geometric form within them, and these objects could be used to achieve a variety of magical effects if removed. In Ultima VII such prisms existed within the (Sphere, Tetrahedron and Cube Generators, which had been erected to assist in the red titan's invasion of Britannia. In the course of forestalling such an eventuality, the Avatar would have to obtain the prism from within each Generator's defenses in order to destroy them. One of these objects, a miniature blackrock cube, had the curious effect of coercing whomever stood in its presence to speak truthfully, which allowed the hero to glean much needed information about Fellowship operations. Eventually, all three prisms would have to be fitted into the pedestals of the Black Gate in order to pierce the barrier that surrounded it.

Later, in the course of  Ultima Underworld II, the Guardian would craft a massive Blackrock Dome over Castle Britannia, trapping all who had been feasting within. In journeying to dismantle this prison, the Avatar would discover a prismatic "echo" of the casting in bottom of the castle's sewer system, in the form of a Blackrock Jewel. Through this gem, the Avatar visited eight different worlds, retrieving an additional reverberant prism from each one. These eight gems (shaped like a small sphere, cube, diamond, large sphere, cylinder, tetrahedron, rectangular cuboid and heart, respectively) would be treated by the court magician, Nystul, before being reabsorbed by the larger jewel—a process that would slowly weaken the structure of the surrounding dome.

Trivia

 * Should the Avatar try to leave without picking up a prism in Ultima VII, then the companions will constantly prompt about it. This was probably done because the prisms are so small und unseeming that the danger of players overlooking them in the badly lit dungeons was too big.