Years ago I discovered that 365 nm light caused fresh eggs to fluoresce. The fresher eggs were a bright red which dulled as the eggs aged. I suspect this is due to oxidtion or it could be the natural UV light in sunlight although many eggs are laid in dark places.
I picked up this very fresh peacock egg last night and took cell phone pictures with while and UV light. The colors with UV were actually much more brilliant that the photo shows but I didn't correct the photo to try to show the true color.
UV_light_on_egg.jpg
I first discovered this by wandering around in the woods with a UV light to see what glowed, and saw a red glow under a cove of brush. I now use this method when examining a found nest of guinea eggs to gather the freshest for incubation while leaving the older eggs to encourage the hens to keep laying in the same spot. Brown eggs tend to fluoresce as well but not as brightly.
JKJ