EXCLUSIVE

Kim Zolciak's left arm is getting a serious workout thanks to the 11 karats worth of ice her husband just put on her for their 5th anniversary.

Kroy Biermann slipped the new diamond on her a few months early, but you know Kim's not complaining. The couple got married on 11/11/11 ... hence the karat count for the gift, we're told.

Our sources say the NFL linebacker dropped $100k on the piece and keep in mind ... Kim was already hauling a 10 karat engagement rock! Modern etiquette says 5th anniversary gifts should be silverware -- but imagine her face if he'd gone that route.

Kroy's playing for the Buffalo Bills this season, so Kim's gonna be solo in Atlanta for a few months.