Hi doma,
I think the law requires platinum.
"(k) The Secretary may mint and issue platinum bullion coins and proof platinum coins in accordance with such specifications, designs, varieties, quantities, denominations, and inscriptions as the Secretary, in the Secretary’s discretion, may prescribe from time to time."
http://www.law.cornell.edu/uscode/text/31/5112
The denominated value of a coin doesn't need to match its metal value. indeed, it is rare if it does. so apparently there's more metal value in pennies than their denominated transaction price.
Same is true for the platinum coin except the other way around.
May require two $1tn coins.
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