There are more missinformation on the care of opal rough and finished stones than in any other part of the opal hobby. Opal is about 6% water. The fear most people have is that it will dry out and crack. In fact under normal usage Australien opal will not crack. Rough opal need not be kept under water until cut. Dealers keep it in water because it gives illusion polish, not because it will crack if it drys. Even the highest quality gems rough is shipped from Australia dry. Finished opals need not to be oiled or soaked in water periodically – although soaking them in water will do not harm. Some people believe that oil protects the color. Actually oil hides the cracks in a stone and can discolor or dull the stone over time. The best care is to wear and love them.
The stories i have heard about cracking fall into three groups. One is the common one of hitting the opal and having a crack. I expect that many circumstances where an opal just cracked are cases where people hit the stone but were unaware of it. – remember the tile nippers! Set properly, this problem can be virtually eliminated.
The second is the situation where an opal has been left in a jewelry
box for several years and is cracked when it is taken out. Again, it may be that the opal was hit by other jewelry over the years. But it may be not the cause. Here, again my recommendation that you wear and love your opal. The moisture the stone get when it is worn may prevent any problems. Alternativly, you could soak it in water periodically . It will not hurt and might help. (Do not , however, soak it if the opal is a doublet or triplet).
The third source of cracked opals is the bank safe deposit box. For some inexplicable reason, safe deposit boxes are death on opals. It may be the atmosphere or a change in pressure or those sonic motion detectors. I don´t know. But i have seen more cracked and ruined opals coming out of the safe deposit boxes than from any other source except abuse (hitting). Avoid them if it all possible . If you do keep unset opals in a safe deposit box, put them in water. Set opals can be placed in plastic bags with a little moisture to help prevent damage.
There is another source of cracking which you should be aware of. Jewelry store windows and display cases use very strong lights which get very hot. So do the opals in display. After a long period of displaying, the opal may crack. I have seen numerous cracked opals in jewelry store displays.
By Paul B. Downing, Ph.D. -“ Opal cutting made easy“