Mixing dirt with acrylics

Hi everyone, 

Experimenting with found objects get my creative gears flowing. My latest idea is mixing gravel dusting into acrylic paint to thicken the paint and create a concrete sort of appearance. This paint will be applied to the wood panel below. This panel was another experiment, letting two colors of wood stain run together and create different accents. Do you have any advice or experience mixing organic material into your paint? I’ll be creating this piece with a time limit, in front of a live audience, so I’m trying to limit my surprises!