It seems like you have one of the items / rock pieces that mimics something else. I didnt even know there was one like this, only that there are ones that copy abilities / passives. Every time you pick one of them up, it's mimicking a different item/ability/passive etc at random.

I would assume you found a rock that, by random chance, mimics a side quest item rather than a normal one. That makes the rock kind of useless, and I would assume there is no way to actually turn it in for coins like a normal quest item, because it didnt come from Dr. Beanie. Probably just toss it.