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 equivalent of Pre-War Money for aliens. It is possible, but unconfirmed, that this is the currency used by the Aliens. Though nominally useless, these can be sold in bulk back in the wasteland for a reasonably large amount of caps. Depending on how hard you search, the cumulative value can reach upwards of 2000 caps.