Minerals like calcium and phosphorus :)
They contain a substance called hardener.
Keratin... it's a natural substance that makes the nail harden.
Bone marrow.
bone marrow
To form into bone; to change from a soft animal substance into bone, as by the deposition of lime salts., Fig.: To harden; as, to ossify the heart., To become bone; to change from a soft tissue to a hard bony tissue.
The substance that makes up the skeleton is called "calcium phosphate," which combines with other minerals and proteins to form bone tissue. Additionally, bone tissue is constantly being broken down and rebuilt in a process called remodeling.
wrist
Bones that are filled with liquid minerals that later harden are called trabecular or spongy bone. These bones have a lattice-like structure that is filled with bone marrow and blood vessels to support bone growth and repair. As the liquid minerals in the bone marrow harden, they give strength and structure to the bone.
bone marrow
MARROW, bone marrow makes blood cells.
Answer
Hydroxylapatite is the mineral substance in bone. It is a calcium compound.