[vlan0]

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.

[Retr0id]

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.

[cameronh90]

Also blood.

[shagie]

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)

[IAmBroom]

Also all particulates.

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