Is it ensure, insure, or assure?
calcium cal·ci·um (kāl'sē-əm)
A soft metallic element that is a basic component of animals and plants and constitutes [approx] 3 percent of Earth's crust. It occurs naturally in limestone, gypsum, and fluorite. Atomic number 20; atomic weight 40.08; melting point 842°C; boiling point 1,484°C; specific gravity 1.55; valence 2.
A silvery-white, moderately hard metallic element of the alkaline-earth group that occurs in limestone and gypsum. It is a basic component of leaves, bones, teeth, and shells, and is essential for the normal growth and development of most animals and plants. Calcium is used to make plaster, cement, and alloys. Atomic number 20; atomic weight 40.08; melting point 842 to 848°C; boiling point 1,487°C; specific gravity 1.55; valence 2. See Periodic Table.