You might want to edit the first sentence in your blurb. A medium is something we know about, it is a common word. Medium stone, on the other hand, is an unknown story-specific artifact. So, it would be better if the sentence reads:
A girl picks up a mysterious rock that grants her psychic abilities.
You also don’t need a comma after ‘but’. Unchecked and uncontrollable duplicate the meaning, and the words look similar, so it might be better to leave just one of them in the description to not scatter the impact of the statement.