Back in the days when I made stained glass art, there was a type of glass that looked like that.
IIRC, it was made by spreading glue over the glass sheet as it hardened, and then the glue would contract and form that type of feathery shape and pull the glass surface with it.
Ah, here it is
I remember when I was a kid growing up in Derbyshire it'd be so cold in our house (we had just a single coal fire in one room) that there'd be frost patterns inside the bedroom windows. I've always loved those patterns.