Here you have one of my favourite collections of materials that is created by White Tiger. Here you find inspiring stone materials as well as a few grass naterial, bark can be used for many other things as well. You can sell things you created with this materials as a part of your createion but you may not sell them as is or in part or part of a collection.
The download pack is aprox 46MByteHere
you can find a bigger picture to see the material balls better!