The windowpane oyster sports a shell so transparent and durable that some Indian and Philippine cities use the coverings as an affordable alternative to window glass. Yet windowpane oyster (Placuna placenta) shells are made up of 99% calcite, a usually brittle material, with a small amount of organic material. A team of researchers wondered how the shellfish’s fingernail-thick armor could survive multiple blows while remaining transparent, a feat unmatched by humanmade materials. When forcefully jabbed with a diamond, the windowpane oyster shell resists shattering by dispersing 10 times more impact energy than raw calcite, the team reports online today in Nature Materials. Peeking at the shell’s crystal structure using an electron microscope, the researchers discovered the oyster’s secret. When stressed, the shell’s crystal structure twists symmetrically, causing an atomic reorganization that forms a boundary quarantining any fractures that might form (pictured). This process, called deformation twinning, dissipates energy horizontally and allows the shell to survive multiple hits. Additionally, sheets of stretchy organic material between the layers of calcite prevent cracks from spreading vertically between layers. The researchers say their work provides a natural template for developing sturdy, transparent materials for windshields and even see-through military armor.