The Cowboys superstar officially hung up his cleats at a news conference on Thursday ... and J.J. thanked Witten for his 15 years of service by giving him a pigskin covered in REAL diamonds.

How do we know it's not a bunch of CZs?? Jones made sure Witten's wife, Michelle, knew they were the real deal -- as if J.J. would EVER miss out on a chance to go big.

Witten finishes his career with 11 Pro Bowls, more than 12,000 yards, 68 touchdowns, a new Monday Night Football gig ... and a shiny new ball!!