I saw this on AMC many years ago, and only one or two scenes. It's new enough to be in color, but old enough that it's the bleeding-eye Technicolor color. :)

There was a couple who were poor, and he proposed, giving her a cigar band as a wedding ring.

Just as soon as he proposes, they get covered in oil... apparently they had a gusher of an oil strike on their land... and in the next scene, they're rich and her ring still looks like a cigar band, but is encrusted in gems instead.

I thought this might be The Unsinkable Molly Brown (because they also use a cigar band for a wedding ring), but the man in that film strikes it rich with GOLD and not oil, and I distinctly remember the oil strike.

Please help if you can! Thanks!