Today, I was red-necking it up with some friends in Auburn at the nearby forest. We found an open area on a decent sized creek that was filled with mostly less-than-fist-sized river rock. After dark, we noticed that when a rock was thrown to the ground (on the other rocks), a small spirt of light would appear. It appeared mostly white, but was perhaps slightly bluish. I know little about geology, but the rocks were of both light color, and consistent in color. What's going on here?