Most objects are immune to poison and psychic damage, rendering them immune to [i]Vicious Mockery[/i] [@Ryonara]. This includes dragon eggs in shady corners, or outside of them at that, but the latter does not really apply here. Also, the target would have to be a creature. And be able to hear you.

You are welcome to use the sling though, and the pebble does seem to hit the oval object at a good angle, some of the force transferring on over instead of simply making a glancing blow at the side. But the sound is not the most promising, it being a click much like you'd tossed a stone at the cave wall, if a little less resonant.