I'd have to say it's a toss up between three different media.

When I was in high school, I made a bonsai planter bowl for a cypress from a cast-off brake drum.  It was so heavy we didn't even have to secure it when a tropical storm or hurricane hit.  Sucker must have been made out of cast iron. 

Another thing i've used is toilet paper. What the hell, I was really broke. I was working on a small piece of painted muslin using watered down acrylic paint mixed with white glue.  When the paint/glue mixture was still wet, but tacky (no pun intended), I wadded up pieces of toilet paper and blotted them on the fabric. This created an unusual texture and also left small toilet paper fibers (cling-ons?) on the fabrics. I then made beads from the finished fabric and sealed them with a very light coating of a water-based polyurethane spray. The muslin was free from worn-out clothes, and everything else was left over from other projects So the jewelry line was essentially a no-cost, recycled project which was very attractive. I marketed it as eco-friendly (which it was), sold it at a local craft fair only 2 miles form home and made a nice profit with a negligible carbon footprint.

 I'd have to say the other most unusual item I used was a large screw and washer and a piece of K-wire (surgical supplies used to stabilized a fracture) which were removed from my knee in 1977. Unfortunately, I lost it a few years after I made the broach!!!! How could I ever replace those supplies.

If anyone has used anything more unusual than that, I'll eat my hat (but not one of my vintage or home-made ones!!

Speaking of recycled materials, we have one of the only paint and solvent  recycling  facilities in the South, or possibly the nation.  If you'd like to start one in your community, google "The Green Project" of New Orleans and they'd be happy to help you get started.