Answer: because the vibration is of very short amplitude, at atomic scale.
Explanation:
The rocks are solid objects.
The solid physical state is characterized because the particles of the solid substances (molecules or atoms) are closely packed in fixed positions.
So, the particles of solids cannot slip (pass) on each other. Nevertheless, yet the particles vibrate rapidly. This vibration is the manifestation of the thermal energy of the substance.
The vibration of the atoms or molecules in a solid is of very short amplitude: it happens at atomic level, so that vibration is not visible. The vibration may be detected by x-ray diffraction but not by visible light.
