Gasoline Sheen on Roast Beef?

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Yup, I believe that Mangetout is on the right track. Alton Brown claims that it’s birefringence, and I’m bound to agree:

Speaking of color, you ever notice the rainbow effect that sometimes appears on the surface of freshly cut tuna? It’s not from age and it’s not fat. It’s birefringence.
[reading from a book] "*Anisotropism of the refractive index which varies as a function of polarization and fre … "

(from here)