It was early on in my work at UNC-G that I posed for the class of a visiting professor (sorry, I don't recall her name). Her proposal to the class was that they paint with dirt - that's right, dirt.
Each student was to bring in some dirt, preferably of different colors. The dirt was mixed with water, just as any pigment is mixed with a carrier of some kind, and applied to large sheets of plywood. Everyone shared, so each artist had a variety of colors on his/her odd pallet.
The final work was to be photographed, since it obviously would not be very durable, would in no way meet "archival standards." The students brought in a surprising variety of colors of soil that day, and I posed just as I would have had they been working with charcoal, pastels or oil paints. The results were quite good, as you can see in the examples provided.