Stone quality defines from how many triangles the wireframe of a stone is built. Using more triangles obviously looks better and is slower. High means a stone is created from 512 triangles, low means from 128 triangles. Why triangles instead of quads? Because most graphiccards can draw triangles much faster.
Any values above 512 don't improve the look anymore but get very slow, so "High" definately means good enough.