Not all creatures have soft squishy eyes like us mammals — the mollusks known as chitons have hundreds of primitive visual sensors all over their body, and they're made of a rock called aragonite.
We've known for a while that some mollusks use calcite for lenses, but this is the first time aragonite has been spotted. These stony sensory organs, according to scientists, "function as small camera eyes with an angular resolution of about 9°–12°." Human vision is about a thousand times sharper.
The eyes are thought to have taken a completely different evolutionary path to the ones that lead to vertebrate vision. They may have low resolution, but these rock peepers do have one advantage: They have two refractive indices, allowing them to focus equally well in and out of water.