Hi Allison, I'm assuming the tub surround will need to be pretty water proof? Any type of tile or rock will work well in that situation. Even wood would do but would need to be sealed heavily to stand up to the moisture.
Here's an idea that might help you get started - https://www.thespruce.com/examples-of-tub-surrounds-1821012
Before one puts tile or rocks on top of drywall, there’s a bigger issue here:
none of those are waterproof at the grout joints. Water damage is serious stuff.
that means, that you’re going to have to make the surface that you mastic these tiles to waterproof.
this can be done several ways, ranging from putting a waterproof membrane on it which you can buy at Home Depot by the lineal foot,
to removing and replacing the drywall at the area you need with either water resistant drywall and then painting it with something like a waterproof enamel paint
or removing the drywall and replacing it with a tile backer board, which is often called Hardi-backer board.
The grout it still should be polymer or acrylic modified.
you can also buy plastic sheet goods that are meant to be waterproof and used in these kinds of situations. some of them are made to look like tile.