The outside of a brown chicken's egg fluoresces a deep red under mercury arc (365nm) illumination. The inner surface of the shell - with the thin elastic membrane attached - fluoresces only white. The fluorescence spectrum has two bright bands: at 620 and 660nm. There is also a very faint, broad band in the green.
The absorption spectrum has a prominent 3-banded structure with continuum absorption shortward of 500nm. Both the fluorescence and the banded absorption spectrum are absent in white shells. (notes 4 Feb. 1979)
eggshell absorption spectrum
The pigment responsible for the colouration is protoporphyrin (C_34 H_34 O_4 N_4). This substance is found in some bones, eggshells and mollusc shells. It is also responsible for the purple colouration in earthworms. (notes 14 Feb. 1981)
As in humans with porphyria, this chemical makes the worms highly sensitive to light and is responsible for the litter of dead worms on the ground when the sun shines after a rainshower. The porphyrins are very stable and have been found in fossil mollusc shells 300-400 million years old.
I investigated these connections after seeing a photograph of fluorescing eggs in an old National Geographic magazine article. I observed the fluorescence myself and looked at the spectrum. I had noted the similarity to the earthworm spectrum before I managed to uncover the porphyrin link from biological reference books. Under a lens, the eggshells are actually coloured flecks of deep purple protoporphyrin in a white matrix leading to the visual impression of a rich brown. Melanain is supposed to play a role in the colouration as well. (notes 4-26 Feb. 1979, 2 Jan. 1981, 14 Feb. 1981). Good references are Vevers (1982, p31) and Fox (1976, 1979)
In my original search for the pigment responsible, I noticed a marked similarity to the spectrum of some flowers, notably a polyanthus. (notes 20 Feb. 1981)
I have also seen the three-banded spectrum in a Blackbird's eggshell - blue with brown speckles. The shell is moderately transparent below 480nm and shows general absorption longward of 650nm - which is probably responsible for the blue colour. (notes 30 May 1981)
Eggshell notebook page
Earthworm notebook page