Metal bits in Bathroom Basin

[PaulMc]

Hi lads, my bathroom refit was finished about 4 months ago, strangely only just noticed as plug rarely in, this fine -- i think metal bits -- in the cold water.
No Tanks unvented system. Took filter off basin mixer tap a Deva and sure enough quite a bit of this stuff on it. Black in appearance don't think its limescale seems like metal but not rusty, Installed using copper pipe, solder joints, new cistern and pan and hansgrohe bar shower, I would be grateful for any help or advice as to what it is or might be and how to stop it," bare in mind all pipework sunk behind tiles".
Cheers PaulMc

 

[PETE 123]

Are they magnetic if so maybe iron, could be plastic if new tank was fitted in loft.

 

[PaulMc]

Are they magnetic if so maybe iron, could be plastic if new tank was fitted in loft.

do you know, i never checked that, however no tank in loft, its unvented system