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I wonder how this pattern formed

Good question, Mila! That's a beautiful and unique opal, so I'm not surprised you scored it since you have a good eye and seem to find ones with special characteristics more often than anyone else I know. Thanks so much for sharing it with us.

Personally I call that metallic mirror where the reflecting surface is clearly displayed. I think the lines are atomic structure, layer ends that create the flagstone. The color starts crystalizing from a point in these on a contact layer and grow into the gel until after they hit a neighboring area coming at them.
With Pinfire it starts throughout the gel. The edges grow until they hit another growing structure. If you slabbed it down like with a triplett wire saw; you could see the changing sizes and shapes of the pattern as you go thru the thing.
And somewhere else i read kitten paws or clover leaf when they are real symmetrical. I would call it a Flagstone.

Thank you!