A British fraudster who charged investors more than $8 million for "ancient slag" -- worthless mining waste -- was jailed in the US on Thursday.

Michael Godfree, 80, sold the tailings from copper mining by the ton, telling victims they contained valuable precious metals.

The Minerals Acquisition Company he co-founded in California provided "certificates of title" with each purchase of material he dubbed ancient slag, and pledged there would soon be a way of getting the precious metals out of the waste.

But the Department of Justice, which prosecuted Godfree, says no such commercially viable process exists, and -- in any case -- Godfree did not even own most of the slag he was selling.

"Godfree was nothing more than a glorified conman," prosecutors wrote in a sentencing memorandum. 

"At bottom, [Godfree] was selling nothing more than worthless dirt (that he generally didn't own) along with a non-existent 'process' to extract value from the dirt.

"Unsurprisingly, not a single victim-purchaser has ever seen any return on their purchase."

US District Judge John A. Kronstadt jailed California-based Godfree for three years, and ordered him to pay more than $8.3 million in restitution.