A spaceship called the Fruitfly, piloted by a character called Fur

I read it as a child and it was one of the best books I ever read. On the cover it had a black hand holding a crystal.

I remember the plot goes as follows:

Four kids, (three boys and a girl, I think) are playing in the woods when they find a crystal being held by a black hand that I think extended from a tree.

They take it, and I remember the hand closed again, stopping them from putting it back.

Somehow they find out it can grant wishes and so they all wish for things. One wishes for a million dollars, and it appears. I remember him stuffing as many dollars as he could into his pockets. The girl wishes for the most powerful shield in the universe. The other boy wishes for a laser gun and the last boy wishes for galactic peace.

A collector robot then appears, and sentences them to hard labour on a prison planet called Amacron 7, or something like that, in payment for their wishes. A desert planet.

The rest of the details are hazy, but they are rescued by an alien called Fur, who liked to eat fruitflies with a drink of Coke. He had a ship called the Fruitfly.

I've made it sound terrible but I remember it was excellent. Any ideas guys?