Harley Behm of Barone Meats in Arizon emailed over the answer to a question about the iridesence that is sometimes seen in cooked meat products like roast beef and corned beef.
According to Harley the phenomena is called IRIDESCENCE. Iridescence is the sheen or rainbow effect that appears randomly on cooked meat products. This sheen is caused by the splitting of light, the same thing that happens when light is passed through a prism. The light is split into a rainbow of color which is easily visible. The potential for iridescence is present anytime in muscle tissue and especially when they contain phosphates or alkyl salts. Muscle fibers act as tiny prisms, splitting light and thereby causing the rainbow effect. Iridescence seems to be more prevalent in products made from the eye of the round. This is due to the density of the muscle fiber. Iridescence is not an indication of bad product nor is it harmful to the consumer.