Wikipedia
Lithophone
A lithophone is a musical instrument consisting of a rock or pieces of rock which are struck to produce musical notes. Notes may be sounded in combination (producing harmony) or in succession (melody). The lithophone is an idiophone similar to the bars on instruments such as the glockenspiel, metallophone, xylophone and marimba.
In the Hornbostel-Sachs classification system, lithophones are designated as '111.22' – directly-struck percussion plaques.