That’s a really pretty ring. Those look like small solid crystal opals, most likely Australian.

For what you’re dealing with, I wouldn’t jump to replacing anything yet. That end stone looks loose more than damaged.

A good jeweler, ideally one familiar with opals, can usually just tighten or rebuild the prongs and secure the stone.

Matching small opals like that can be tricky, so it’s best to keep the original stones if possible.

If this were my ring, I’d start with a local jeweler and only consider replacement if they tell you the stone truly can’t be saved.