The difference is in the minting of the coins. Copper has become so valuable (and pennies so, well, not valuable) that it ceased to make financial sense to put a lot of copper in pennies after 1982. They could make them out of zinc and they'd still look the same to most everyone. But anyone trying to use the special properties of copper would notice.
Copper is better than zinc at binding oxygen, and that's how the whole thing works. Oxygen plus hot acetone burns red-hot, and the penny is catalyzing the approach of the gas to the liquid. Zinc just doesn't stick to oxygen as much as iron, and it does not burn the nail polish remover.
Of course, this requires a bunsen burner and a flask rather than a magic wand and a cauldron, but it works for me all the same. Plus, I can actually do it. Do I have a volunteer from the audience?