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Calcium carbonate (CaCO3)
from wikipedia.org
Birds are known for their hard-shelled eggs. The eggshell is approximately 95% calcium carbonate crystals, which are stabilized by a protein matrix. Without the protein, the crystal structure would be too brittle to keep its form. The standard bird eggshell is a semipermeable membrane filled with thousands of pores and covered on its outer surface with a cuticle, which helps the egg retain its water and keep out bacteria. As an egg develops within a bird's reproductive system, the shell is laid down as a liquid mineral layer around the already-formed extraembryonic membranes. This takes place in a part of the bird's oviduct which is called the uterus and serves a very different purpose from the uterus of a mammal. The shell is laid down by shell glands. In an average laying hen, the process of shell formation takes around 20 hours. Pigmentation is added to the shell, coloring it any of a variety of colors and patterns depending on species. The waxy cuticle is deposited on the shell in the vagina of the bird's oviduct. The chick has an egg-tooth which it uses to start a hole in the hard eggshell to allow it to hatch