Diamond dust has been found in sediments across North America from 13,000 years ago. It's not much of a find in terms of riches: rather, it's an indication of what may have happened 13,000 years ago. This is about the time that many large animals went extinct and the Clovis people declined, along with a start of a 1,300 year long ice age.
The discovery of diamond dust lends support to the theory that there may have been a comet that hit North America 13,000 years ago and caused widespread devastation. Diamond is merely carbon arranged in a certain crystalline lattice formed by high temperatures and pressures. For diamond dust to be found in a single layer of sediment, the most likely explanation is that the impact of a carbon-rich comet compressed carbon into diamonds. The presence of elements such as iridium in the same layer of sediment is another piece of evidence supporting the comet impact theory.
Some scientists are skeptical about the evidence supporting the comet impact theory, however, so it still might be the case that the mass extinctions 13,000 years ago were purely due to terrestrial causes. Still, it doesn't stop me from worrying time to time about a comet wreaking terrible havoc on our puny human civilization.