Michael Irvinsays someone stole one of his favorite pieces of jewelry -- a diamond necklace with TONS of sentimental value -- and he's mortified that it's now gone.
The Dallas Cowboys legend lamented losing the piece to his social media followers on Wednesday ... saying the ice was jacked by a thief in Miami last weekend.
Irvin says when he was returning to his hotel from a Univ. of Miami football event ... he set his black Louie Vuitton bag down -- which had his necklace in it -- to interact with some fans.
Irvin claims while he was sidetracked ... a man grabbed the bag and bolted.
Michael pleaded with his followers to help find the scumbag and return his stuff, but the former NFL superstar seemed to believe Wednesday there was no hope of getting it back.
And, Irvin said he was straight-up depressed over it, because his chain was PRICELESS to him.
"I have to come out of this depression. So I am sharing with ALL," Irvin said. "This is my necklace that was in my bag. I designed it as a first down chain, holding an all diamond football, which is holding an all diamond cross."
"On each end of the cross are diamonds I acquired at the birth of my children Myesha, Chelsea, Michael and Elijah. It was a piece I built over my life that represents. Faith, Family and Football and my own sentimental way of praying to GOD please always protect my babies."
Irvin added, "I am so hurt its gone."
It's unclear if authorities are still on the hunt for the thief. Irvin had earlier offered a reward for any information regarding the heist.