Exactly as it says. How about an option for patina'd copper and rusted iron sheetmetal blocks? Purely aesthetic, possibly created by running a copper/iron sheetmetal block through a bottling machine with water, or possibly the mixer with water. Perhaps random textures or CTM for both? Though an easier option may be a random overlay, similar to the TBA mud dungeons from Betweenlands (as it has normal blocks but with mud overlays randomly applied to blocks.

Examples of the mud dungeon blocks
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normal blocks

Vanilla has corrosion for copper, and I'm not adding it for iron.

Too little value for the effort involved.

Why not have exposure to running water cause it and then you have to combine sand + paper to create sandpaper to remove it