I was rolling an ingot of silver, and soon discovered it to be full of air bubbles.
Usually this is rejected and would be recycled.
I decided to keep rolling it out anyways to see what would happen...
As you keep flattening it out, the bubbles get squashed too, and become lamina (layers).
Then they split and lift.
Then it kind of looked like birch bark, all wrinkly white with black striations.