the “skeleton” is made of what kind of material?

The human skeleton, often referred to simply as "the skeleton," is primarily made of bone tissue, a composite material consisting of an organic matrix (mainly collagen) reinforced by inorganic hydroxyapatite minerals (calcium phosphate crystals).

Composition Breakdown

Bone is approximately 30% organic material by dry weight, dominated by type I collagen fibers that provide flexibility and tensile strength, much like steel rods in reinforced concrete. The remaining 70% is inorganic, chiefly hydroxyapatite (\ce{Ca10(PO4)6(OH)2}), which grants rigidity and hardness, enabling the skeleton to support body weight and protect organs.

Key Components

• Organic phase (∼30-50% by volume) : Collagen (90% of organics) plus proteoglycans and glycoproteins for elasticity and resilience.

• Inorganic phase (∼50-70%) : Hydroxyapatite crystals (39% calcium, 18% phosphorus by mass) aligned along collagen fibrils for optimal strength.

• Cells and water : Osteocytes (up to 15% volume) maintain the matrix; water adds about 25% volume.

This living nanocomposite evolves through remodeling, adapting to stress as seen in athletes' denser bones.

TL;DR : The skeleton's "bones" are a tough mix of flexible collagen proteins and rigid calcium-phosphate minerals (hydroxyapatite).

Information gathered from public forums or data available on the internet and portrayed here.