"This jet black wafer hums quietly, and seems to quiver with potential energy."
Antimatter Cells are one of the types of Energy Cells available in Caves of Qud.
It is currently the longest lasting Energy Cell in the game.
As an unknown Artifact, it may appear as "small trinket" or "small cube".
It is worth looking out for those cells when trading with high level dromad traders, especially in the energy cell slots of the items they sell, because antimatter cells are a steady source of the rare metacrystal crafting component.