A lot of sculptors use resin mixed with bronze powder (known as bronze-resin or, sometimes, cold-cast bronze).
I use only stone-resin. Wherever my sculpture is labelled ‘bronze’ it means foundry-cast bronze.
Stone-resin is polyester resin mixed with stone-dust. I use four stones of different colours, ground down to different sized dusts or grits and mixed together in varying proportions.
The base colour comes from a welsh grey limestone and the dominant red speckle comes from a scottish red granite. The other two stones are a cornish green granite and a white italian limestone (used in varying proportions to grade the depth of the final colour).