If necessary, use a very mild detergent and a soft brush to clean a solid opal; rinse in clean water. Avoid solvents such as alcohols as they may dry the opal. Wipe doublets and triplets with a damp soft cloth but never get them wet. Never use any abrasives on opals, even a mild toothpaste. Opals are very prone to scratches. They have a hardness similar to glass, so don't use anything to clean your opals that you wouldn't use on your sunglasses or eyeglasses. Even paper towels can cause minute scratches in opals, so use a soft microfiber or cotton cloth to clean your opals. Never use an ultrasonic cleaner, as this could crack or shatter your opal.
If your opal is scratched or has lost its shine, it has to be resurfaced and polished. This should only be done by an experienced opal cutter or jeweler. Avoid these problems by wearing your opal in situations where it will not be subjected to excessive wear or damage. Opals should not be worn as everyday jewelry unless you are prepared to restore or replace them periodically. Extreme cold or heat can also cause opals to crack.
Store your opals in moderate humidity. Some suggest storing opals in glycerin but this can cause a sticky mess that itself is difficult to clean up. Store your clean opals in a sealed plastic bag with a cotton cloth that is dampened with a few drops of water. The cloth does not have to be soaked, just slightly dampened. Most storage vaults have very low or zero humidity, so be sure you use a plastic bag if they are going to be stored in these areas. Attics can reach high temperatures and low humidity--two things to avoid when storing opals.