As the vibrator/tumbler media does it's "thing", some of the kernels naturally break down into dust. You get range dirt, firing residue, primer dust, and whatnot imbedded in the kernels as they polish.

I wash my media in hot, soapy water every few weeks. You would be surprised to see how much crud and **** comes out in that dirty water!

Replacing is the easiest of course, but washing, rinsing, and drying out in the sun takes little effort and makes the media like new again.