Most gems stay the same. Ethiopian opal changes with the water.
Opal forms when silica gel fills cracks in rock and slowly hardens as the water leaves. In Wollo, that rock is volcanic, and the opal that filled it was only discovered in 2008.
Hydrophane opal is porous. Soaked in water, it turns more transparent, and its weight can rise by as much as 10%. As it dries, it returns to its original look.
That same porosity is why it deserves care. Put it on last and take it off first, and keep it away from water, oils and other liquids, which can change its colour.