This gem dealer’s tax returns were as phony as a cubic zirconia.

The owner of a Midtown jewelry business pleaded guilty this afternoon to cheating the IRS out of more than $200,000 by stashing millions in Swiss bank accounts.

Richard Werdiger of Michael Werdiger Inc. faces up to 2-1/2 years in the slammer under terms of a deal with Manhattan federal prosecutors.

Werdiger, 63, also agreed to pay a fine of at least $3.5 million and an as-yet unspecified amount of restitution.

Werdiger — who was busted on Tax Day last year — told the judge that the case has had a devastating impact on “myself, my family and my business” and that he was “prepared to accept the consequences” at his June sentencing.

He declined comment afterward.