About 100 species have these eyes, and while they seem rudimentary at face value, they see well enough to help the mollusks protect themselves from passing predators. Unlike most eyes, they're not made out of proteins. They're inorganic, made out of the same aragonite (a kind of calcium carbonate) as the rest of the shell.
“What nature has perfected is to use comparatively simple, cheap starting materials and turn them into an exquisite, multifunctional material,” Peter Fratzl of the Max Planck Institute (who wasn't part of the study) told Science Magazine in praise of the research. “We want to copy that approach.”
Close study of the rocky eyes revealed some ingenious engineering indeed: The lenses of these eyes feature larger grains of aragonite than the rest of the shell, and they're aligned differently. The differences make the rocky material better suited for vision, but they make the eyes weaker than the rest of the shell, too. It's a fascinating balance, because the presence of the eyes make the shell vulnerable, but the material of the shell makes the eyes less effective. The tools that these chitons have to work with aren't perfect — but they've found a good compromise. That's exactly what engineers hope to copy using similar materials in the lab.
For a more detailed report of the study, check out Ed Yong's coverage at The Atlantic.