Does anyone know what this is?

[Petra]

[abyssunder]

I suppose it could be a sandstone concretion, possibly with an iron compound.

Edited April 13, 2022 by abyssunder

[hemipristis]

Bog iron.  

Bog iron forms from the oxidation of reduced iron around a nucleus.  The reduced iron (Fe+2) is dissolved in water. A change in redox conditions causes the iron to oxidize to Fe+3 and precipitate out, typically entrapping sand with it in the process.  It is the exact same process that produces iron staining in the sink and toilet

[Petra]

Thank you so much!