There are lots of materials and strategies for building contact microphones from piezo discs. The discs are fragile, and need some modification to work well in field recording applications (strength, shielding, durability, water resistance, etc). Audio recordings (normalized) form the basis of a qualitative comparison of different materials and their application.
A contact microphone (piezo disc) needs a special preamp to prevent the attenuation or loss of low- and mid-range frequencies in the sound signal. There are some DIY and plug/play options available. I bought a couple of modules from Stompville and installed them into plastic jars for use in field recordings of everyday objects.