Please understand that I can’t see this opal, or test it etc, but it sounds like a possible imitation to me. There are many Glass copies (with gold flecks) which are called ‘Slocum Stone’ in the Trade.

I suggest you take it to your local FRIENDLY Jeweler or better yet a Gemologist, who should be able to confirm it one way or another.

At least then you would know for sure and can stop guessing