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[ASO]

If your not wearing a opal regularly the most important thing to worry about is storing it in humidity not necessarily water.  If a Ethiopian opal is stored in water it can actually help it to crack, I have seen this happen personally.  Something I have seen in Australia was the effects of very high end stones being stored in vaults that were designed to preserve important documents buy removing humidity in an already arid environment and all the moisture got sucked out of the stones drying them out and cracking them.  In a situation like this a opal should be placed in a sealed container with water in a vault and I believe this is where allot of these myths stem from.  In most parts of the world on any given day the air has enough humidity in it to not cause any problems with quality opal.

[gemfeller]

Absolutely do not store opals in bank vaults without precautions as mentioned.  The dehumidified air is almost certain to craze stones.

As mentioned, "most" climates allow opals to aspirate any moisture needed for stability that's lost due to evaporation.  But a change of climate can damage some stones. I cut a nice opal for a gold pendant I made for my mother many years ago.  When she passed it went to my daughter.  It had been stable for many years until she took a job in far southern Arizona.  In just a few days the ultra-dry air caused it to craze.

The difference between "healthy" and "cracky" opal is a fine line.  I spent a few days mining opal years ago with the owner of the Rainbow Ridge Opal Mine in Virgin Valley, NV.  We worked in an underground shaft where the opal was found in a very wet montmorillonite clay where it remained stable.  He showed me how freshly-mined pieces began to craze immediately when exposed to sunlight -- you could hear the tinkling sound as the crazing proceeded.

But stable and cuttable opals from the same source could be found by winnowing mine spoils that had been exposed to the elements outside the shaft for several years.  The crazing appeared to be caused by too-rapid evaporation of water from the stones. But buried in the spoils, some of the opals seemed to lose water slowly enough to avoid crazing and stabilized. While the vast majority of VV is cracky, it illustrates a larger point about opal in general.  The secret of stable opal appears to be a balancing act of retaining a precise amount of water, and resupplying any lost to evaporation in the exact amount needed.