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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).

marked as duplicate by Otis, TheLethalCarrot, Virusbomb, Jenayah, DavidW Jul 16 at 13:43

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    Is it by chance any of the several answers under scifi.stackexchange.com/q/66578/4918 "Children's novel: three children (plus dog?) travel to the Moon in pretend spaceship turned real" ? – b_jonas Jul 16 at 10:00
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    You helped me identify the book with the link. thank you. It is: "Wonderful Flight to the Mushroom Planet." I will need to read the book again before I can be sure that the colored gemstones were in this book or another. Perhaps I need to wait for you to make a comment with the answer before I select it as the correct answer? – museful Jul 16 at 10:23
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    One of the more obscure stories in the series is "Jewels from the Moon", maybe you are remembering that? You can read about the series, and indeed, download and read the books here: peyre.x10.mx/MushroomPlanet/index.htm – Organic Marble Jul 16 at 14:22
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    Museful, "Return to the Mushroom Planet" included the colored stones. A bad adult (who may have stowed away on their ship? I can't remember) took the valuable stones from the mushroom planet. However, in the moonlight of earth, they looked like nothing, and he threw them down on a beach (as I recall). He was under the influence of an amnesia potion he had taken. After he discarded the stones, the protagonist kids carefully picked them up again. – CreatedByBrett Jul 16 at 18:53
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    Gems from the Necklace of Ta feature in a later book in the Mushroom Planet series, "A Mystery for Mr. Bass". Each regular person on Earth who possesses one of the gems becomes obsessed with mushrooms in some way. – nebogipfel Jul 16 at 22:47