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Most likely, pre-1990.
This is a story (book for kids) about two kids that build a spaceship. I remember the spaceship being constructed of shiny metal or tin on the exterior of the ship. I also vaguely recall them using a hammer to attach it. It was built from common materials. I believe in a garage.
It wasn't particularly realistic, and the ship should not have been able to fly.
I also remember that they were helped or joined by a alien (I think he was green). He helped them prior to blasting off into space. He might have been a neighbor.
I also vaguely recall colored stones or gems (perhaps I am mixing stories up).