There are two main types of bone tissue: compact and spongy. The compact bone makes up the outer layer of bone and is much denser than the spongy bone, which is found in the inner layer of bone. The two types of bone tissue have different functions and are made up of different materials.
The most significant component of compact bone is hydroxyapatite, a mineral made up of calcium, phosphate, and carbonate. Hydroxyapatite gives compact bone its strength and hardness. The other significant component of compact bone is collagen, a protein that gives the bone its flexibility.
The most significant component of spongy bone is collagen. Spongy bone also contains small amounts of hydroxyapatite, but it is not as dense as compact bone. Spongy bone is much lighter and more flexible than compact bone.