(FORTUNE Magazine) – The first $100 U.S. coin is hot off the minting presses. Don't go spending it at your local minimart, though--it's worth more than it seems. About $430, in fact.

The coin is meant as an investment, says U.S. Mint director Philip Diehl. And while the government has issued investment-grade coins before, this one-ounce newborn is different. It is the first U.S. coin to be made out of platinum. It's the first U.S. coin to sport a "$" symbol rather than spell out the word "dollar." It's the first coin to boast its own purity, 99.95%. Most important to investors, the Platinum American Eagle is the first coin to be eligible for IRA plans--starting Jan. 1, 1998--thanks to this year's Taxpayer Relief Act.

The coin is sold by banks and brokerages at a 4% to 6% markup over the market price of platinum, which as of Oct. 10 was $429.

Why platinum? Its value is up 15% this year, while gold is down 10%. It's the rarest precious metal around, 15 times scarcer than gold and 125 times scarcer than silver. And demand for platinum, which is used in everything from catalytic converters to liquid-crystal displays, has doubled in the past 15 years.

--Maria Atanasov