Transinium was a mineral.
Transinium could be identified by its appearance as a black rock with a dotted scattering of iridescent crystals. It was also very brittle.
Though it was generally viewed as a low-grade fuel, it was known to amplify some non-nuclear reactions, but it was very unstable. When given enough power, transinium completely decomposed very rapidly. (TNG video game: The Transinium Challenge)
In 2364, the crew of the USS Enterprise-D investigated a terrorist attack possibly related to the mineral. A sample received from a Ferengi trader had aqua-colored crystals. (TNG video game: The Transinium Challenge)