What is the hardest organic material?

The teeth of Patella vulgata, a species of sea snail that is widespread in the Mediterranean Sea.

The measured breaking strength values reach 6.5 GPa, which is higher than the 4.5 GPa of spider silk. This species has about 2,000 of these razor-sharp teeth, which it uses to scrape food off rocks. Below are the teeth seen under a microscope.

Limpet teeth are the strongest biological material ever discovered.

These considerably high values exhibited by the teeth of this mollusk are due to two factors.

The first factor is the scale of goethite nanofibers (an iron hydroxide, the material of which they are made of). At this length scale, the materials become insensitive to defects that would otherwise decrease the breaking strength.

The second factor is the critical fiber length of goethite fibers in limpet teeth. Critical fiber length is a parameter that defines the fiber length a material must have in order to transfer stress from the matrix to the fibers during external loading. Materials with a very short critical fiber length (relative to the total fiber length) act as effective reinforcing fibers that can absorb stresses on the matrix.

Goethite nanofibers have a critical fiber length of about 420–800 nm, which is several orders of magnitude shorter than…