Bad luck to wear an opal if they are not your birthstone. I wear a black opal set simply in gold. It looks exactly like a Mood Ring and years ago was often confused with one. Beautiful blues and greens depending on the light. I have another bought in Sydney in the 70s. It too is a "black opal" but has yellow, green and red flashes.
I love them. Never had done anything special to care for them.
My first opal was set in a dinner ring and was stolen - long story. It is cursed. Wouldn't take it back on a bet.