Great question- there’s a couple of things that control this. Firstly the shape of the crystal- the crazier the shape the more electricity will be generated. You also want it to be flexible and elastic- because the more you can bend/press it the more electricity that is made. The other important thing is called permeability- this is how fast the electricity can travel through the crystal. The materials I use, like proteins, have really low permeability which means the electricity can travel quickly from the crystal into devices and makes them better than devices made from things like lead.