Following the graphite microphones failure, we made our own contact microphones. The main idea is to grow crystals that have piezo-electric capabilities, as described by Leaf Cutter John here.
The recipe is for Rochelle Salts and everybody can make it in the kitchen:
The actual crystal growing took the night, while the whole process took around an hour.
Some examples of the crystals.
And the crystals are quite nice too.
With two wires on two opposite sides of a crystal, one can retrieve an electric signal proportional to the mechanical deformation: a contact microphone! Below a test with a music box:
The music box and the crystal beneath it: