Black velvet on magnet after rebuild

[worn]

It seems that as I look back on builds that after break in I have removed a sump plug with it’s magnetic tip only to fine a black velvet-like sludge on the magnet. This has happened on more than one occasion. The motors have run well, but I am still wondering what it means. The material is black and magnetic. It sort of turns into paste when you rub it off. And it is the same color and texture as the black finish on new or reground camshafts. Any ideas about if I oughta worry? Not bearing material I am sure because it indeed sticks to the magnetic plug. Seems associated with new motors though. Anyone know what it is, or whether to worry?
Thanks.

[Trackrash]

That is why the oil is changed soon and often after a rebuild.

I would also want to look at the screen on the bottom of the motor.

[dannobee]

Rings.

[worn]

Nothing in the screen. I seem to recall seein his on my cast iron engines as well. Rings would make sense, although I was focused on rockers. Thanks.