Effect of using a polarized filter. The rock was a varnished black sand stone. The two photos are a shot a few seconds apart. The top one has been shot with the filter rotated in an angle that doesn’t have any effect on what you see (just slightly darker). The bottom one has been shot with the filter rotated in a way that it filters out all the reflected light.
At the bottom you can see the difference after the two photos have been rotated and aligned. Cool huh?