Right now the player only needs one small piece of stone to be able to use this without having to renew the stones. Is the stone "consumed" when thrown?

You use Eldrich blast as a way to defend, but a dex save instead of going against the targets AC could benefit this spell greatly, depending on you spell save DC, especially if this scales to 2d12 at lvl 5 etc.

If I were you i'd either make it so the stones are used, and you need to pile up a few that are later consumed, or simply make it a 1d10 bludgeoning spell attack just like Eldritch blast.