What determines the color of eggshells?

Eggs have shells that are mainly (about 95 percent) composed of calcium carbonate -- a white substance that you have no doubt also seen as chalk or limestone. There are additional chemicals that can affect color.

The breed of chicken determines what color the eggs are, so it's a question of genetics. In a way, it's not so different from what determines the different colors that peoples' eyes or hair can be, even though they are all people.

Brown egg shells get their color from a substance called protoporphyrin that comes from the breakdown of hemoglobin in blood. This pigment is deposited on the surface of an otherwise white egg as the egg is formed and can be rubbed off with sandpaper or dissolved off with vinegar -- something you can easily verify at home. Blue and green colors come from a pigment called oocyanin that is produced in birds along with bile. As you point out, there are actually chickens (Araucana chickens) that lay blue-green eggs. They are sometimes called ``Easter Egg" chickens since their eggs are precolored.

Assuming that the chickens are well fed and healthy, there is no particular reason to expect that one or another color egg is more or less healthy. Rumors that Auracana chicken eggs are lower in cholesterol have not been supported by tests.

Incidentally, if you look at various other birds, there are other pigments that can appear as egg colorings, turning eggs brown-red (oorhodeine), brick red (lichenoxanthine), or various shades of blue or green.

Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819. Include your initials and hometown.