Tiompan your a clever man. Ask a music teacher since I am one I'll see what my memery brings. Arnhill has been used in some pieces to do with Celtic Connections, I've heard that on the web somewhere. Kilmartin has been used in a similar fashion with stones being used as instruments. The most famous lithophone I've seen is in Keswick Museum, The Musical Stones Of Skiddaw. Most recently I watched Paul Anderson perform his new work which uses North East stones. Old keig has fantastic acoustic properties. Kilmartin would be considered an instrument as there are more than one, The Ringing Stone, Arnhill, Roddanstone if struck would be more of a gong. However Skiddaw uses scales. In the far east the gamelan music style uses stone lithophones like glockenspiels, similar uses have occurred in India and Japan as well as South America. So I would say that they probably are lithophones as you can change tone by the various beaters used and the force you strike the rock. Similar to drummer using sticks, brushes, reeds, nylon heads etc.