That appears to be bubbles inside, not mica.
I really think this could be a Master Cloudy.
If that's so, then it's a relatively uncommon. Well, I don't know how common it is, but probably most people who have them don't know what they have. I had some when I first started collecting and didn't know what I had and sold them cheaply. I don't know what value they should have, but it should be quite a bit more than an ordinary solid-colored marble.
If it is indeed a Master Cloudy, I'd call it a keeper.