I am a cook technician.
Recently I hear in a meat store a conversation of the seller (P) with a woman (G):

Do you have marble beef?

(P): Yes, we recently brought it cheap, but we threw it out.

Why were they thrown out?

(P): No one took it from us, and they didn't take it home to cook because when we cut it there were many thin fibers of fat and all sorts of foods.