Yes as soon as i saw your heading i was going to warn you that Opals are very soft.Opals rank at about 5 or 6 on Mohs scale of mineral hardness.
Tips for buying an opal engagement ring
•Pick a rub-over (bezel) setting. This is pretty much a "must" for opal engagement rings, as rub-over settings provide much better protection and security for your opal. A thin bezel of gold follows and covers the edge of the stone, protecting damage from impact, and ensuring the stone stays securely in place. Claw settings are much less secure, provide little protection, and can wear down over time - especially in rings.
•Boulder opal is harder wearing. Due to its very hard natural ironstone backing, Queensland boulder opal is more robust and has an advantage over other types of opal. For an opal engagement ring, boulder opal is ideal. Black opals, crystal opals, and white opals are also suitable, but do not have the same hard-wearing quality as boulder opals. Due to their unusual 'free' shape, boulder opals also lend themselves to more creativity in design.
•Select a stone with a low cabochon (i.e. dome on top). Opals with a high cabochon are more exposed and vulnerable to impact damage, so if your stone has a flat or low cabochon top, it's less likely to be damaged.
EDIT Warning...Things like going from a warm ambient room temperature to a very cold temperature sometimes cause opal to crack.Opals are like sponges. They expand and contract with heat and cold and that's why they can crack easily.
Just be very careful if you get Opal ok :)
Good Luck
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EDIT 2- Well if your into vintage,why not find a vintage setting? My fiance got me a diamond ring but the setting is vintage and everyone says how much they love the designs on it.You should google Vintage Engagement Rings and see what im talking about :)
And you say that White Opal would match anything? How about White Sapphires?
A White Sapphire Ring in a Vintage setting?