10:00 AM PT -- Along with no ring, we've learned Cardi's no longer following Offset on Instagram ... and he's responded in kind.

Cardi B is leaving one stone totally turned -- her giant engagement rock is off her hand, and she's making sure everyone knows it.

Cardi strolled into Madison Square Garden Friday night for iHeartRadio's Jingle Ball concert and proudly showed off her much lighter left hand to fans and photogs.

No doubt, she's sending up a signal -- to her estranged hubby, Offset ... and possibly to all the future suitors waiting to shoot their shot. Y'know her DMs are lit, right now.

Just last month, she was rocking the 8-carat, $550k ring while onstage for a Pandora event in NYC. Things change.

Earlier this week, Cardi announced she and Offset were separated and likely headed for divorce.

If she's busted up about it, she's doing a great job of hiding it in public. Ever the professional, Cardi shook off any Offset-Summer Bunni drama ... and ripped through her set at MSG. No ring required.