There are fake acorns, fake snakeskins, fake malachite clock and fake bronze pendant. It's amazing what can be done with polymer clay.
Polymer clay is a sculptable material based on the polymer polyvinyl chloride (PVC). It usually contains no clay minerals, and is only called "clay" because its texture and working properties resemble those of mineral clay. It is sold in craft, hobby, and art stores, and is used by artists, hobbyists, and children.
You can vote on your favorite challenge piece on this page of the Polymer Clay website. Voting ends at the end of March.
My favorite is a grouping of pendants shown here, which are meant to represent carnelian, bone, turquoise and leather. Which is your favorite entry?