I am sure your massage technique works, but not by piezo-electric charges. Whoever sold you that explanation was trying to blind you with pseudo-science.
Put very simply - since you were rather dismissive about it some way above - piezo-electric charges are triggered in certain materials when they are under changing strain; and vice-versa, but only while the strain is changing and then only in certain directions. The loudspeaker in your 'phone is probably piezo-electric, as is the igniter on many gas ovens.
Those materials include quartz, common in many but by no means all types of stone; but the chances of a random pebble of quartz-rich rock actually producing any electric charge when you tap it, are extremely tiny.
I would suggest the massage works simply by the heat and contact of the warm stones, not any sort of electrical phenomena, though I wonder if telling the client about the supposed piezo-electricity gives an extra placebo effect to the treatment.