Black Particles in one toilet in the house

[Petragnani]

I am on well with pressure tank and have a water conditioner. Recently, in the most used toilet, there are black particles at the bowl water line. This material is removed when the toilet bowl is cleaned. There are two other toilets in the house where this doesn't occur. I'm not sure if it is a problem with the well water (high Iron) or is the bladder in the pressure tank coming apart?

[jadnashua]

Pop the tank cover and look in the tank of the toilet. Check out the flapper valve. Is it black? Feel it, is it coming apart?

[Petragnani]

Doesn't have a black flapper valve. I was mistaken. The black particles appear in the other two toilets in the house but on a less frequent basis, again at the water line in the toilet bowl.