Tunguskite is the result of a small planet in orbit around a black hole , exposed to extreme amounts of Hawking radiation. The crystals are described by Mary Jo Irving in Lifeline: Crisis Line as off-white crystals that generate their own inner light. They are also said to grow on their own, expanding and then splitting in two.
Tunguskite is incredibly valuable and is sought after by the Pearson Corporation, because it is the only mineral capable of efficiently containing negative energy.
A small amount of Tunguskite crashed on Earth, and this impact was the cause of the Tunguska Event.