Calendenium is a naturally-occurring chemical substance.
In 2367, an alien ship carrying calendenium was trapped in a Tyken's Rift. It was determined that combining the calendenium with hydrogen would produce an explosion which would free itself from it. The USS Brattain and USS Enterprise-D had also become trapped, and the Enterprise was able to supply the hydrogen, successfully creating the explosion. (TNG: "Night Terrors")