Though this topic may not be directly related to materials engineering, it tells us how nature’s creations are sometimes amazingly shocking. I hit upon this topic while reading ‘The Oxford Book of Modern Science Writing’ by Richard Dawkins. In a creature like Trilobite whose evolutionary period dates back to the Lower Cambrian period and is one of the oldest known arthropods which is believed to be extinct now, there existed one of the most complex eye structure ever. Trilobite eyes are made of calcite crystals, the purest forms of calcite being transparent. The crystallography and optical properties of calcite have been exploited to the hilt in these animals. Take a look here for more explanation.