Tooth enamel is not as hard as even unhardened steel. It's a composite material whose hard phase is hydroxyapatite, a hydroxyphosphate of calcium, which is even more brittle than hematite. But the hydroxyapatite crystals are below the critical size for flaw-insensitivity, and the protein matrix that binds them together is too soft to propagate cracks from one to the other. Limpets' teeth do use goethite, an oxyhydroxide of iron, in exactly the same way, achieving hardnesses that do exceed iron's.

So in a way we do already have iron teeth because there's an iron compound better than calcium compounds for teeth, if "we" includes mollusks.