What does a evolution stone do when held in a battle?

Question

I wanted to know because I've heard that it boosts the power of a pokemon that isn't at its last evolution form.

Answer 1

It raises a NFE (Not Fully Evolved) Pokemon's Defense and Special Defense by 50%, similar to how a Light Ball doubles Pikachu's Special Attack and Attack.

http://www.serebii.net/itemdex/evolutionstone.shtml