The Migos rapper dropped $550k on Cardi's engagement ring, which took a month and a half to make ... according to the owner of NYC's Pristine Jewelers. It came together just in time for Offset's live onstage proposal, but means he really must have been sweating during their brief breakup.

Cardi's shiny 8-carat stone's surrounded by 2 half-carat pear diamonds and 2 carats worth of pink and white stones. The ring's totally custom made in the raindrop shape Offset wanted.

It also worked out perfectly -- we're told the ring was finished by the day of the Philly Powerhouse concert and he picked it up just hours before he proposed.

As we reported ... Offset could get a big chunk of his ring money back -- he and Cardi B are wanted for a TV wedding special that could land the couple one million bucks.