Finger for non-engagement ring

Factual Questions
1

This is almost a IMHO:

On which finger would an unmarried (American) woman customarily wear a ring that wasn’t an engagement ring?

Backstory: My gf’s birthday is coming up and, for various fairly irrelevant reasons, we both think that a Celtic themed ring would work well. We’ve been together awhile and the ring would carry an implicit message of “we’re serious,” but we don’t want to commit a faux pas.

I’m sure this topic has come up previously, but my google fu is weak today (can’t find the right keywords).

2

Typically anything worn on the right hand is not indicative of relationship status. So then it just depends on the size of the ring relative to the size of her hands - where does it look best? But if this could be construed as a “promise” ring, it could easily go on the traditional left ring finger as well.

3

A ring on any finger (or thumb) other than the left “ring finger” is not a symbol of anything – it’s just jewelry.