I need help with the title of a late 70's early 80's sci-fi movie about space aliens.?

I remember seeing this movie as a kid and can not remember the name of it at all. I want the say that the name was something like Master Blaster (but it is not the Master Blaster movie that they made with the people in the swamp). OK, an alien crashes in the desert and a young man late teens early 20's finds the body and ship. He takes a stone that is on a rope from around the alien's neck and a tube/gun of some type that fits over the aliens arm. He takes these items home and puts them up except for the stone which he starts to wear around his neck. His chest begins to become hard (think like a callus) and he becomes evil. Once evil he puts the gun on his arm and goes on a rampage killing and destroying. Eventually 2 other aliens come to earth and you find out that they are cops or bounty hunters and they take the boy because he is now evil. I remember that the aliens at the end of the movie were claymation like Clash of the Titans. Anyway, any help would be appreciated.