An engagement ring is worn on your hand pretty much all the time. Opals are easily damaged, diamonds are exceptionally durable.

Hardness absolutely matters in rings. Not as much in pendants or earrings, but people don't realize how rough they are with their hands. Most people do not take their rings off to wash their hands, or do their laundry, or, or, or. So many things have unexpected abrasives that may just feel a little rough on your skin, but can significantly damage a soft stone like opal. In a rush and accidentally bang your hand against the door frame? Chipped opal. Back of your hand itches, so you rub it against your jeans briefly? Scratched opal. They're very fragile stones.