Alien crystal

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

About
Alien Crystals come in two variations; Small Alien Crystals and Large Alien Crystals. They are commonly found aboard Mothership Zeta.

Though nominally useless, these can be sold in bulk back in the wasteland for a reasonably large amount of caps, as well as a weightless and fairly plentiful source of Rock-It Launcher Ammunition

One might assume they are an alien form of currency. much like Pre-war money, as they are both weightless and can be converted into caps very efficiently.