Eggshells are composed almost entirely of calcium carbonate crystals, stabilized by a protein matrix. These proteins keep the structure of the eggshell from becoming too brittle. Bird eggshells can dissolve in various acids, including cooking vinegar.
Inside a bird egg is a yolk surrounded by albumen, commonly known as the egg white. Two shell membranes, the inner and outer, located just beneath the eggshell, contain the albumen. Eggshells protect the egg from damage and contamination as well as regulate the gas and water exchange that occurs throughout the development of a bird embryo. A thin coating on the eggshell called the bloom or cuticle prevents dust and bacteria from passing into the egg.