T.J. Watt Balls Out For Fiancee's Engagement Ring, Massive Bling Worth $100K!

T.J. Watt CLEARLY knows diamonds are forever ... 'cause the NFL superstar went all out for his fiancée's massive engagement ring -- scoring a piece of ice worth $100,000!!!

The Pittsburgh Steelers linebacker popped the question to his longtime GF Dani Rhodes at a beautiful waterfront spot last week ... and the ring he used for the occasion is AWESOME!!!

TMZ Sports has learned ... the 26-year-old hit-up celebrity jeweler Moe Diamonds for the bling, and he spared no expenses.

We're told the ring features a 2.5-carat flawless colorless triple EX graded diamond -- which is very rare -- and has 18k of white gold with 1.33 carats of VVS-flawless diamonds in the band.

All total, we're told the ring is valued at $100K!!!

And, what's even more interesting about the ring ... we're told Watt hit up Moe way back in June 2020 for the piece -- so he's been planning this for over a year!!!

"It still feels like I’m dreaming," Rhodes said on Instagram the day after saying "yes."

"Woke up today feeling so incredibly happy and blessed that I get to marry my best friend🤍 I can’t wait to love you FOREVER."

Watts and Rhodes have been dating since 2016 -- when they met at the University of Wisconsin.

They're both professional athletes ... Watt is entering his 5th season with the Steelers and Rhodes just signed with the Throttur Football Club, a soccer team in Iceland.

In fact, Rhodes is former teammates with J.J. Watt's wife, Kealia -- the two actually played together on the Chicago Red Stars women's soccer team.