You have to be careful cleaning opals - they hate rough treatment. Gentle soap and water should do the trick.
With the water.... check the back of the opal. If it is very flat it might be what they call a doublet where an opal is placed on top of another material and glued with resin. They hate water so should only be wiped with a damp cloth.
You can ask the jeweler who does your valuation to clean it for you.