Coloured scanning electron micrograph (SEM) Chicken eggshell surface (Gallus gallus domesticus). Shown here are the surface mineral crystals that form many tiny pores in the bird eggshell. Eggshells are the outer covering of hard-shelled eggs and of other eggs with soft outer coats. Eggshell structure varies widely among species. Birds and some reptiles produce hard-shelled eggs. Bird eggshells are composed of a protein matrix lined with mineral crystals, usually calcium carbonate (CaCO3). Most of the eggshell is composed of calcium carbonate crystals (approximately 95%). As the chick develops changes in the eggshell allow the passage of carbon dioxide and oxygen and the release of water vapour by way of the eggshell membranes. Magnification: x4, 000 when shortest axis printed at 25 millimetres.