Alien crystal

Alien Crystals are items which are added to Fallout 3 in the Mothership Zeta add-on.

They come in two variations; Small Alien Crystals and Large Alien Crystals. They are commonly found aboard Mothership Zeta and are the equivilant of Pre-War Money for aliens.

Trivia
The Alien Crystal could be a slight reference to the intro from the original Half Life, in which the scientist protagonist Gordon Freeman inserts a similar Alien Crystal recovered from the boarderworld Xen into a machine, causing an inter-dimensional rift to open, spawning an alien invasion.