I recently agreed to repair a friend's 1968 banana seat coaster-brake bike. The bike had been in their basement for decades, but the friend wanted to ride it again. (Her height hadn't changed much since when they got the bike as a a pre-teen.) I liked the thought of seeing an adult tooling around on a vintage banana seat bike, so I agreed to do the labor for free. I assumed that the inner tubes would need replacement, but I had my friend stop up the sink anyway, just so I could do the usual water test to check for leaks. But no sooner had I got the tubes off the tires than I knew that the test would not be necessary. There was something hard, crunchy, and rattling on the inside of the tubes.

I took the tubes into the LBS for replacement. Shorty was so curious that he took a knife and cut open one of the tubes. Inside were several pieces which looked like hematite rock. Apparently, this is what some of the plastic on the inside of an inner tube can do after sitting forgotten in the basement for over 3 decades....

Since carbon black is a component in butyl rubber, and hematite is often found around coal, this is plausible. Probably was actually some iron oxide (Fe O3) rather than hematite.