Eggshells are made almost entirely of calcium carbonate (CaCO3) crystals, which make up about 95% of the shell. The remaining 5% of the shell is made up of hundreds of different proteins that affect how calcium carbonate crystallizes. Eggshells are semipermeable membranes, which means that air and moisture can pass through their pores. The shell also has a thin outermost coating called the bloom or cuticle that helps keep out bacteria and dust. The inner and outer membranes lying between the eggshell and egg white provide efficient defense against bacterial invasion. The average eggshell contains about 0.3% phosphorus and 0.3% magnesium and traces of sodium, potassium, zinc, manganese, iron, and copper. If the calcium from the shell is removed, the organic matrix material is left behind, which has calcium binding properties and influences the strength of the shell. The majority of the true shell is composed of long columns of calcium carbonate, and there are other zones that are involved in the self-organization giving the eggshell its strength properties.