These are naturally occurring grains of platinum, similar to the gold placer grains one finds when panning for gold in streams. The seller says these are especially rare because many people who pan for gold throw them back in the water thinking they are just ordinary pebbles. They certainly look ordinary, but their high density would make them settle out with the grains of gold, which are similarly dense.
Source: eBay seller prospectorsgold
Contributor: Theodore Gray
Acquired: 23 November, 2003