Good ... 'cause it's hanging around Aaron Donald's neck -- a $250,000 diamond chain that's helped give A.D. the power to be the NFL's top defensive player this season!!!

Shortly after the Rams DL inked his record-breaking $135 MILLION deal in August ... Donald nabbed this custom chain from Moe Diamonds in Pittsburgh.

The ice is INSANE ... TMZ Sports has learned the Miami Cuban chain alone is approximately 3/4 kilo of 14K gold with over 60 carats of true graded VVS-VS colorless diamonds.

In the AD/99 pendant ... we're told there are 25 carats of true graded VS1 diamonds -- with VVS diamond encrusting the whole thing.

We're told the bling took 2 MONTHS to put together ... and appraised at $250k -- though sources say Donald paid "an undisclosed amount" for it.

Whatever A.D. ultimately spent -- it's been more than worth it ... dude leads the NFL in sacks this season with 14.5 and is EASILY the favorite for Defensive Player of the Year.

BTW ... the Donald chain is Primetime approved -- Deion Sanders couldn't help but drool over the piece when he saw it a couple weeks ago!!