Coffee Mugs Cups

Can you tell me anything about the dark rings inside of Ceramic Coffee Mugs?

When I take my coffee Cups from the dishwasher they have dark circles inside. What is that? Is it toxic? Has the coating worn off by the coffee? Should I throw the mugs away or are they safe to drink from?
The mugs are clean, no coffee stains. If the porousness question were true, wouldn’t it be equally distributed? The rings are black/gray not brown like coffee. I think there’s something else going on. And I’m not looking to change my coffee drinking habits. I suspect the lines I’m talking about would be present in the cups of tea drinkers as well.

Those dark rings in your coffee cups exactly match the ones on the lining of your stomach. Presuming you drink the same coffee.