If it’s one ring and it’s diamond (solitare, three stone, eternity, etc.) and not very cocktail-like, I assume it’s an engagement ring.
One colored gemstone ring is not necessarily much more ambiguous. If it is a colored gemstone ring and otherwise similar to a typical engagement ring (in size and style), I will lean strongly toward assuming the person’s engaged. Same thing for a carved all-metal ring, e.g., I know of one that features 3 carved metal flowers in the shape and size of a typical diamond 3-stone engagement ring.
Very large or loud rings, be they decorated with diamonds or colored gemstones or neither, I find confusing, and the less the ring has in common with the traditional types of engagment rings, size and style wise, the less likely I am to assume it’s an engagement ring.
All the above assumed the ring is made out of fine materials. If the ring is made out of fashion materials, I assume it’s a fashion ring.
Things get a lot clearer when there’s a wedding band.
One solid precious metal band by itself says “married” to me.
Two rings, one engagement-type ring (categories #1 & #2 above) on the outside, and one band on the inside, says “married” to me just as much.
A less engagement-like ring (cat #3) on the outside and a band on the inside suggests “married” fairly strongly, even though by itself I’d have found it much more ambiguous.
… I guess overall I’d say my assumptions are based more on to what degree the ring is “engagement-y” overall (classic style and size, fine materials, the addition of a wedding band on the inside) rather than just diamond or no-diamond. Yes, diamonds are still strongly the norm, but if somone has, e.g., a .75 carat ruby solitare in gold, paired with a gold band on the inside of her finger, I’m not at all confused as to what it means.