Black grit in toilet

Hi all, hoping I can get some help here.

My mom has a toto toilet, about 5 years old in a house of the same age. In one of the toilets she gets a black grit build up in the very bottom of the bowl. It's not a stain type thing, it's litteraly gritty. There is a problem with the toilet running she just told me about so I'm going to look at that today to see if maybe the flapper needs to be replaced. Someone told her this grit is likely mangenese deposits. I thought that would show up as a rusty/red stain? My mom said she can fairly easily scrape the grit loose with her nail.

Something I should add is that there is a sediment filter on the main line that needs to be changed. My dad passed away suddenly in November and that just wasn't top of mind. I'm going to change that but I'm also going to look at the screens in the taps. I asked mom if she did that and she hadn't so I figure I should expect to see the deposits in the screens too.

It's weird that it's only one toilet, but what I'm thinking is that if it's "running", there's always a bit of water coming in with no flush to move it out and since the filter needs to be changed, it could all be linked.

Does this make sense? Any thoughts?

thanks!

Yes it does make sense that a running toilet would catch a load of grit.

ok well the whole house filter is replaced, boy did it need it! and the flapper is replaced and adjusted so it's flushing well so we'll see what happens. what annoys me is that it appears that the finish on the toilet is gone or something, in the very bottom, it's black that won't come off (or so my mom told me, i didn't test that). what i also found interesting is that the water seems to be "Eating" the flapper. it was actually bubbled in spots and gummy. it's well water so no chlorine and the water was tested to chester county PA standards a few years back (we're in wayne county PA but chester is much more strict apparently) and all was fine but of course, something unregulated might cause this. i'll have to keep an eye on it.

If she ever used one of those tablets or anything else that sits in the toilet tank, that will quickly degrade the rubber parts.