Unprocessed opals are considered unstable. The water that gives them their fire may or may not evaporate when exposed to air. For this reason, unprocessed opals are ALWAYS stored in plain water. It's a matter of let the buyer beware. Stable, unsealed opals are considered 'better' than the sealed ones but the custom is to let the buyer/cutter take the risk of finding out if the stone is naturally stable or not. Of all the gemstones, it's probably the most fragile, both physically and chemically. Tanzanite, by the way, is hard but brittle. Never use it in a ring, bracelet, etc. I spent about a decade in and around the fringes of the business. This is gleaned from both texts and general lore. Feel free to PM me for specifics.