It's just the fat/grease. And yeah, the reason it's a rainbow is exactly the same as what you get from an oil slick.

I've seen it on beef that was grass-and-hay-only fed on my family's ranch and butchered by a local shop, so I doubt it has anything to do with modern industrial processing.

I found an interesting explanation of the phenomena that points to diffraction of light due tiny water droplets present at the cut ends of the muscle fibers (note the effect is seen on meat that has been cut across the muscle fiber grain like in deli meats).

http://www.madsci.org/posts/archives/2001-09/1001732824.Ch.r.html

I've noticed that too. And I am a roast beef aficionado. I have also noticed that is most likely on the cheaper roast beef that I only rarely buy.

I almost always buy applegate roast beef at the central market. Easily the best I've found at a convenient (sorta) to shop at place. It NEVER has that sheen. It also cost about $15/#. I occasionally buy the cheaper stuff that almost always has that sheen and costs about $8/#.

The big differnce isn't the fat content or even the cut of beef used. I've read the ingredients on the wrap on both packages. The applegate stuff ,iirc, adds little or nothing to the beef. The cheaper shit adds some sort of a solution to the beef to add flavor or preserve it or something. And that sheen is almost always most prevalent in those types of roast beef that are "italian flavored" or some such nonsense.

So, while I think it's plausible that fat or grease plays a role, and Lifting Missives et al excelent contributions notwithstanding, I can't help but feel the solution used is at least partially responsible for the particualr kind of sheen I believe you are referring to.