Originally Posted by Trsnrtr

Brake block dust.

Originally Posted by noodle soup

at the stem? I doubt that’s what it is.

Originally Posted by noodle soup

It’s just dust.

I see it on almost every bike I service(including my own). I’d be willing to bet that there’s also some near the BB and hubs. It doesn’t take much petroleum residual to collect dust.

Originally Posted by wgscott

I think this is correct. It definitely gets up in there. I had to tap my fork steering tube out with a rubber mallet, so I did it over a (nice) chair, and there was a pile of brake block dust. This is with disc brakes, not rim brakes, BTW.