The answer is that the rainbow sheen on meat is "completely normal" and safe to eat.

Occasionally, beef exhibits a rainbow sheen when cooked due to the phenomenon of light diffraction. This phenomenon occurs randomly and is not influenced by the conditions in which the cattle were raised or the methods used to store and process the meat. This is the conclusion of the United States Department of Agriculture (USDA).

Specifically, when you cut a piece of beef, light interacts with the fat and iron on the cut surface, causing a diffraction effect that bends the light rays, creating a rainbow sheen. The USDA research also points out that this phenomenon can occur not only with beef but also with pork, fish, smoked meat, ham, etc., due to the reflective properties of these meats when exposed to heat.

Researchers indicate that this phenomenon is very normal and can occur with pork and fish as well.