The rainbow, metallic sheen on deli-style roast beef is a harmless phenomenon caused by the diffraction of light on the meat's muscle fibers.

It can also be seen on meats like ham but is more noticeable on darker meats due to their color. White meats like chicken may not display it as prominently.

The rainbow effect doesn't affect the meat's quality and is purely a result of how light interacts with the meat's surface.

Deli meats, like smooth-cut varieties like roast beef, are often iridescent due to their ultra-smooth surface created by mechanical slicers, exposing more meat fibers to light.

Cured meats, such as pastrami, are more prone to iridescence due to added moisture from phosphates in the curing process.