The hardness of a material determines its resistance to permanent deformation. When considering biological structures like bone, the concept of hardness becomes complex because bone is a living, composite material rather than a simple, homogeneous mineral. To understand how bone compares to other substances, scientists often look to the Mohs scale, a system originally designed for classifying minerals. This scale offers a limited framework for assessing the scratch resistance of bone’s primary components.
Understanding the Mohs Scale of Hardness
The Mohs scale of mineral hardness is a qualitative ordinal scale developed in 1812 by German mineralogist Friedrich Mohs. This system measures a material’s resistance to scratching or abrasion, not its absolute strength or density. The scale ranks minerals from 1 (talc) to 10 (diamond).
Testing is performed by attempting to scratch a material of unknown hardness with one of the ten reference minerals. If the unknown material is scratched, it is softer than the reference material; if it remains unmarked, it is harder. The Mohs scale is relative, meaning the difference in hardness between consecutive numbers is not uniform. For example, the jump in hardness from corundum (9) to diamond (10) is significantly greater than the jump between any other two consecutive numbers on the scale.
The Hardness of Bone’s Primary Mineral Component
Bone’s rigidity comes primarily from its inorganic component, a calcium phosphate mineral known as hydroxyapatite. This mineral forms tiny, needle-like crystals that make up approximately 65 to 70 percent of a bone’s total mass. These crystals are responsible for the bone’s compressive strength, which is its ability to resist being crushed.
When tested in its pure, crystalline form, hydroxyapatite is assigned a Mohs hardness rating of 5. This places it directly alongside the reference mineral apatite, which is number 5 on the scale. Materials with a hardness of 5 are considered moderately hard, comparable to glass or a pocket knife blade. The hardness of human tooth enamel, a highly mineralized form of hydroxyapatite, is slightly higher, reaching a rating of 5 or 6.
Why Mohs is an Imperfect Measure for Bone
Applying the Mohs scale to an entire bone is misleading because bone is a sophisticated composite material, not a simple mineral. It is composed of hard, inorganic hydroxyapatite crystals embedded within a flexible, organic matrix. This matrix is made largely of the protein collagen, which accounts for about 25 to 35 percent of bone’s mass.
The Mohs test only reflects the scratch resistance of the hardest surface component, the hydroxyapatite. It fails to account for the softer collagen fibers, which provide elasticity and tensile strength—the resistance to being pulled apart. This combination of hard mineral and flexible protein prevents bone from being brittle and shattering like a pure rock crystal.
Because the Mohs scale only measures scratch resistance, it does not provide a complete picture of bone’s mechanical performance, which also involves strength and fracture toughness. In materials science, researchers instead use quantitative methods like the Vickers, Knoop, or Brinell scales. These tests measure indentation hardness, which involves pressing a shaped indenter into the material to calculate its absolute resistance to permanent deformation under a precise load.