Damian Rzeszowski showed no emotion in the dock as the presiding judge, Sir Michael Birt, read out the verdicts that two jurats - similar to magistrates in the UK - had reached.
Throughout the trial, the defendant had constantly stared down at the floor, not looking up at any time.
No members of the victims' families were in court, but the public gallery was full of curious onlookers.
Judge Birt, who is the Bailiff of Jersey, said Rzeszowski would be sentenced on October 29.
Two metal detecting enthusiasts in Jersey have dug up a treasure trove of coins which could be worth up to £10m.