Apparently the designers wanted using the Samsung Galaxy S III to feel like a "walk in the forest", with all the natural sounds around you, like the chirping message notification, and so on. They wanted to represent a wild boar attack when you try to change the launcher, but this got canned in the last minute.
We kid, but the "marble white" or "pebble blue" in the color description stems directly from one of the designers' observation of pebbles lying in a creek, with the water flowing over them. Overflowing city pools, light reflections off of multiple layer coatings and the impression of pouring various liquids have all come into play when designing the phone to look and feel organic.
Whether you can call a polycarbonate material "inspired by nature" is an altogether different story, but things still have to be light, durable and signal-allowing, so using it on the huge 4.8" phone makes sense.