In general, you should know that any fine material that is chemically inert respective to all the other Resin components and that adds desired physical properties to the finished product can be used as filler. Examples are:
1)Sand, sand dust
2)Dust from carbonated rocks (e.g. Marble). These have to be used in non acidic resins only!
5)Glass fiber shreds
6)Paper and cardboard dust or shreds