Investigating the composition of sand from around the world is a fairly common activity in many earth science or geology courses. So it wasn't unusual (as a science teacher who subscribes to the NSTA listserv) to see someone ask for help identifying a particular mineral in a sample of sand.
In this case, the request was made by the operator of The Science of Sand asking about these greenish nodules seen below from a sample from Andalusia, Alabama:
After some investigation, it looks like the greenish nodules are most likely the mineral Glauconite, a complex silicate, K2(Mg,Fe)2Al6(Si4O10)3(OH)12.
OK, so that's mildly interesting. But it gets better.
Glauconite granules are fairly common in quartz sand and limestone throughout most geologic ages, but are particularly common in Cambrian age deposits. Why? Because.
Glauconite granules are trilobite poop.
From History of the Earth:
To put it bluntly, glauconite pellets are trilobite poop. OK, not just trilobites, and that’s not the only way glauconite forms. But the little round grains in marine rocks are thought to be an alteration from the original fecal pellets excreted by marine organisms. It can also precipitate directly, and it can form when some iron-bearing minerals are weathered, but the pellets in sandstones are generally accepted to represent fecal material.
Trilobites weren't just a passing fad in Earth's history. They were common animals for their entire 270+ million years roaming the floors of the ocean. In some places, the trilobite poop is so thick it shows up as a distinct layer, as in the lower right of the picture below showing the Cambrian Lion Mountain Sandstone:
So, next time you go for a walk in the sandy beds of the Conecuh River of Andalusia, Alabama, just think about how great it feels to have trilobite turds between your toes.