Tyler Herro is just a rookie, but he's turning into a vet baller right before our eyes ... by copping an insane diamond-filled chain ... and it's worth a whoppin' $55k!!!

The 19-year-old ex-Kentucky star has been a stud in his first year in the Association ... he's currently averaging 13.4 points a game and has already gotten praise from ex-Miami champs like Chris Bosh.

TMZ Sports has learned ... Herro wanted to immortalize his own "TH14" logo in bling, so he hit up celeb jeweler Iceman Nick to make it happen.

We're told Tyler got hooked up with a 14k gold pendant with 25 carats of VS1 white and blue diamonds ... and an iced-out 14k rose gold Cuban chain with 18 carats of diamonds.

Add all that good stuff together and you got a piece worth about $55k!!!

FYI -- Nick has a huge list of clients ... from folks like Errol Spence to Deshaun Watson to like, 1/4 of the Buffalo Bills roster.