Would be cool to see some more variation and deeper normals in those rocks. Some cracks and divots. Really depends on the type of stone though.
Also would look better doing it as an alpha over the rock like you were asking about.
For some really good rock references try
http://www.rockclimbing.com/ and browse there photo galleries. The scenic section is quite nice.
Once I get some free time I am going to put together an archive of rock textures. The few crags I hit consist of basalt, quartzite, granite, sandstone and lime stone.