This part is specifically designed to make the moire/tree ring effect, caused by inaccurate microstepping or otherwise insufficient horizontal resolution, visible. Use this to test for it so that you can recalibrate to get rid of it. You probably won't need to worry about it unless you are trying to print layers .1mm or smaller.

You don't get the full effect on the round pieces from the pictures, but check out the full-size (which is also upside-down for some reason) to get a better look: http://thingiverse-production.s3.amazonaws.com/assets/7e/1e/bb/0e/66/Photo_Jun_12_12_40_03.jpg

The effect will be easier to see when your vertical resolution gets better than your horizontal resolution. The print shown is at .1mm, and I calculated the rings to be caused by an effective horizontal resolution of .17mm (with proper calibration of my pololu drivers, it should be 1/16 of that).

To make the effect most visible, print with shiny filament at .1mm or better vertical resolution.

More information on this effect: https://plus.google.com/u/0/105535247347788377245/posts/F8qQT2GB8q5
https://plus.google.com/u/0/105535247347788377245/posts/N6JqqFd4hF9
https://plus.google.com/u/0/105535247347788377245/posts/RuyyVcKSZi3

Additional reading: http://hydraraptor.blogspot.com/2009/08/motor-maths.html
http://hydraraptor.blogspot.com/2012/04/stepstuck.html