In 1997, the United States began striking platinum coins to compete with other world bullion coins.  U.S. platinum bullion coins are available in Uncirculated and Proof versions in four denominations: $10, $25, $50, and $100. The true value of these coins is tied directly to their intrinsic metal value, although in a few cases (where mintages are low) collectors will pay a significant premium over the bullion value.  Generally, however, the bullion value of these coins far outstrips their face value.

Proof coins may be ordered by the general public directly from the Mint, either as single coins or as a complete set of the four different denominations.  Uncirculated coins are distributed by selected representatives, who then make them available to the secondary market based on the prevailing bullion value of platinum.

John M. Mercanti designed the obverse of the U.S. platinum bullion coins using the head of the Statue of Liberty.  In keeping with the Mint's "American Eagles" theme, new reverses are developed each year - always with an eagle incorporated into the design.

The metal content of each denomination consists of 99.95% pure platinum.

Mintmarks appear on the reverse, but positions vary from year to year.  U.S. platinum bullion coins have been struck at the following mints:
Philadelphia 
West Point

BEWARE - Counterfeit examples of the 2002 1/10 ounce platinum coins began being offered in 2003.

Denominations:
1/10 Ounce Platinum - $10 
1/4 Ounce Platinum - $25 
1/2 Ounce Platinum - $50 
1 Ounce Platinum - $100 
Four-denomination Platinum Proof Sets

Sources and/or recommended reading:
"Counterfeit American Eagles available at eBay auction site" by Paul Gilkes, COIN WORLD, December 29, 2003