It's called hematite and its very widespread. Also, rust reacts with concrete as is, though not as much as with water. Hence why you'll see rebar in those plast

It's called hematite and its very widespread. Also, rust reacts with concrete as is, though not as much as with water. Hence why you'll see rebar in those plastic sheaths sometimes. My SO does a lot of work with concrete and these effects are well studied. Turns out that concrete isn't super stable over kiloyear times, let along megayear times. I think you'd have a really hard time trying to conclusively say that some bit of rock was fabricated 10k+ years ago, as compared to all the other types of rocks out there. https://en.wikipedia.org/wiki/Hematite https://en.wikipedia.org/wiki/Hematite https://www.cement.org/learn/concrete-technology/durability/corrosion-of-embedded-materials https://www.cement.org/learn/concrete-technology/durability/...