Ice ball update

I've become rather addicted to the ice balls I described earlier. I now make them with mineral water, though I don't know how much difference that really makes. They are certainly not crystal clear—I think you have to buy in elaborately made professional ice to get that. In fact I added one to a glass of bourbon last night and noticed some rather pretty patterns in the ice:

Rather like a tiger's eye gemstone, isn't it?

Can any science boffins out there tell me what is going on here? I'm guessing that as the water freezes from the outside in, something happens to trapped air, but I don't think I've ever seen anything quite like it before.

Or like a frozen exploding sun. There's a good name for a cocktail