Derek Jeter -- KING OF THE DIAMOND ... Check Out the Engagement Ring

Nice rock.

Here's Hannah Davis at the CMAs Wednesday night ... showing off the insane diamond engagement ring she got from her fiance Derek Jeter.

Looks like a round-cut stone with a Pave band or something ... and probably very expensive.

Also, make sure you take your attention away from her finger and check out the rest of her body at some point ... good lord.

Congrats, Mr. Jeter.