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Aluminum nuggets from bonfires or past trash-burn pits.
The reason why they might not *seem* to be aluminum at first, is that when they are molten down into the nuggets, they are so unfamiliar to our normal feel of aluminum. Which is always lightweight things like the thin cans, thin tabs, foil, etc.... We're not used to holding aluminum in "nugget" form, so we don't expect them to be heavier than the usual aluminum we're accustomed to handling.