Name of a science fiction book?

its me

New member
I read a book that I think was from the Young Adult section of the library, years ago.

In it, the characters found this set of 6 large stones in the ground (like large paving stones), and when they walked on them, it transported them to another planet. The new planet had 2 moons, and unicorns. The cover art shows a young man sitting on a large black unicorn. Upon using the stones again, they traveled through time.

I would love to read this book again, if anyone knows the title or author, I sure would appreciate it.
 
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