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 c ... (full story)