You could use papier-maché, Model Magic, Paperclay, Wonderflex...various materials that you could mold by hand to give that organic look. There are also faux-finish paints designed for home decor that give a stone effect.
COSTUME-CON 32 - 2014
The International Costuming Convention is coming to Toronto Canada!
4 Days! 4 Masquerades! Dealers, Tours, Parties, Costuming Panels and Classes, Cosplay, Props, Doll Show & Competitions, Steampunk, Sci-fi, Fantasy, and so much more!