Thanks for all your replys guys. But I am still confused a little. FvM, the objective is to actually charge using a piezo-crystal. I have a crystal, I connected it to a digital voltmeter and it showed some 3.2 to 5 V when I applied some pressure on it (with my thumb). Now consider that this is kept in a shoe at a proper position so that such pressure generates some voltage as a person jogs/runs. Why would it not be useful to charge the mobile battery which is some 3.7 V?
Freebird, if I use a supercap, would I be able to "store" the energy in it and then later use it to charge the battery? Even that would be good. But how do I connect the supercap exactly and what more circuitry would be required? How would the entire circuit look, apart from it looking like a piezoelectric generator.