Pretty freaking cool! These are Fresnel Lens from old Big Screen TVs.
Obsidian artwork and metal projects:
Yeah, since the invention of the magnifying glass there's been probably a billion or so ants who found out real fast how hot focused sunlight can get...
I used to engrave things on pieces of wood with a magnifying glass, usually South Park characters (they're easy to draw). You'd need welding goggles with sunglasses underneath, it was so bright.