Why is sliced roast beef iridescent?

Answer 1

Its totally normal and its called birefringence.

Here's the deal. The cut surface of meat is pretty smooth. But, if you look closely, you'll see it's actually a vast landscape of parallel fibers. When cut, the reduced surface pressure coaxes microscopic beads of moisture to the surface, each one of which acts as an independent prism. When viewed in concert, they look like a rainbow.

Answer 2

I couldn't of said that better myself "scott"

: )