Dear Student,
When two stones strike against each other then spark is produce due to friction between the surface of the stones. But striking of stone cannot be considered as slow combustion.
Reason: Combustion is the process in which a substance combines with oxygen to produce heat. Combustion is an exothermic process i.e., the process is accompanied with the evolution of heat. The release of heat may or may not result in the production of light. Thus, two important things required for the process of combustion to take place are a fuel (combustible substance) and supporter of combustion (oxygen)
Stones only produces spark on striking they do not undergo combustion as stones are incombustible thing.
Regards