Ah I see. It's the arrow keys to move the boulder. Left and right move it left and right. Up spins it and down drops it quicker. There is a little tip showing that in the bottom right of the screen but maybe that isn't obvious enough. Thanks for letting me know!
Thanks for playing! That's so strange I've never hit and issue with controlling the boulder at all. Just to double check you're on the level with the green gem and once you've collected the gem and a new boulder spawns you can move it with the arrow keys?
If so can I ask if you're on pc or Mac? I'd love to try recreate this and get a fix sorted!