Copper Crystals on a Penny 

This happens because of a displacement reaction. You have a solution supersaturated with a copper salt, and you add a metal ion that is more reactive than the copper, which steals the cation from the copper salt, leaving a bunch of disassociated copper ions in solution. If you drop the copper penny into it, it provides a nucleation site for the copper ions to deposit themselves, producing what you see in the image. (Source)