Would it be possible for biological life to evolve a part of itself to be as hard as iron? I’m thinking of a tree analog. The wood or bark can replace metal for a low-tech society. The planet itself is deficient in metals because of its history, so I’m not thinking that the life form uses naturally-occurring iron as part of its makeup, even if that were deemed possible. Just a variation on bark/wood that is this hard. Or at least as hard as bronze, significantly stronger than wood on Earth. What do you think?