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I was guessing petrified dinosaur stomach lining, prototripe, as it were.

Rub them briskly on a cloth and see if they smell sort of piney. Better, but slightly destructive, is to heat the end of a paper clip and press it into the scale where nobody will see. If they're...

For a little excitement, put on sunglasses and take your old rotten celluloid outside and burn it on the sidewalk or some other safe surface. It's as bright as magnesium! Pretty scary to think of...