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Hmm not just coffee stains too. If you've ever had a water leak on gypsum board, the edges of the water ring are darker.

I think that's a similar but different effect, as the water travels outwards from the centre due to capillary action it pulls particles with it.

Also blood.

Side bit from some recent news about dried blood and its crackle pattern... https://phys.org/news/2025-04-blood-droplets-inclined-surfac... (saw it https://news.ycombinator.com/item?id=43852446 though it didn't get too much attention)

Also all particulates.

It isn't about the chemistry of the suspended/dissolved solids.