ruby said:

I have rock questions MV.
One of my nursery buddies took pictures at Bouddi National Park of a couple of interesting rocks and was pondering on how unusual they are. I said ‘I know just the person to ask about them!’. He sent me the pictures, and now I can ask you for all the interesting facts :)))

Both photos show ironstone concretions in sandstone.

Iron travels through rock in low temperature water solutions in the Fe2+ (soluble, reduced) state. If the fluid encounters a change in oxidation in the rock (many reasons this can occur), iron can be oxidised to the Fe3+ state, which is insoluble. The iron drops out of solution as iron oxides and iron oxy-hydroxides (effectively rust) deposits.