Mollusk shells are tough - they easily withstand most attacks from predators. Yet, the shell of the windowpane oyster is also transparent. This odd combination has caught the attention of biologists for years, but researchers have just uncovered the mystery of their unique structure. This new discovery could signal the arrival of a new age of eyewear. 

Researchers from the Massachusetts Institute of Technology (MIT) subjected these mollusk shells to a diamond tip press. They used this tool to create indents in the target and examined the marks using an electron microscope and other instruments. 

Ling Li, a graduate student at MIT, along with Professor Christine Ortiz, noticed fracturing broke the shell into pairs of crystals. These diamond-shaped calcite structures exhibited mirror images, like the wings of a bird. 

Windowpane oyster shells were seen rebuilding damage using an atomic process known as twinning. This process involves two neighboring crystals growing together in specific molecular configurations. 

When this occurs, it brings about "a series of additional energy-dissipation mechanisms... which preserve the mechanical and optical integrity of the surrounding material," Li said.

The reconstructed areas of the shell are also ten times more efficient than the original material at dissipating energy.  

Mollusk shells are composed of around 99 percent calcite, which if fractured are easily broken. It is the one percent of organic matter contained in the shell that gives each one its unique properties. For Placuna placenta, the result is a shell which transmits about 80 percent of the light that strikes it. 

These transparent shells may have evolved as a means for the mollusk to keep an eye on its environment while still maintaining a tough exoskeleton. 
Windowpane oysters have often been used as a substitute for glass, and the material is often called cadiz. The first known use of the mollusk shells as home windows happened in China.

Bioceramic armor of mollusks could become the basis of a new generation of eyewear for both soldiers and the general public. Lenses could repair small scratches on their own, and last longer than current materials. For emergency workers and those on the battlefield, the material could be used for blast shields and visors. Even windshields for automobiles could, one day, benefit from this research. 

"As a first-of-its-kind [demonstration of] the effectiveness of deformation twins in natural materials, this work should have huge practical impact," Huajian Gao, Professor of engineering at Brown University, not involved in the research, said in the press release announcing the result.

Study into the nature of the armor of mollusks was published in the journal Nature Materials.