One can only assume it's an engagement ring, right? It certainly looks like one... or rather several ones melded together to create A SUPER RING. In short, it's a rather sizeable ring. Here's where we'd usually insert a 'massive hewp' joke but it's a bit obvious at this point. 
Minaj posted the below to Instagram captioned "Now this is what I'm talking about baby. Lol. Love u." She then added: "RANG finga whr da rock iiizzzzzzzz"

For those requiring another angle...

According to People, Minaj first sparked engagement rumours earlier this year in April when she was spotted wearing bling on her ring finger. At the time, Mill confirmed they were not engaged via Fader: "(Our relationship) is definitely real, but it ain't really time to get married yet. We're still learning each other, feeling each other out."

And the feeling is Minaj is fond of the diamonds.