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Sapphire is mysterious. At low T, DMF is slow enough to be observed directly as a “relaxation” of the diamagnetic signal. The fraction of muons participating in this process is reduced by a small positive electric field, without a concomitant increase in the stable diamagnetic component, suggesting that a positive E enhances the initial Mu fraction. This makes no sense unless the muon stops preferentially behind its own radiolysis electrons (patently absurd) or the effect of E is to suppress the formation of diamagnetic Mu- in favour of neutral Mu. Diamagnetic RF resonance experiments on n-type Si have revealed the dominance of a Mu- state at high T, so this is not unprecedented; but it is unknown why Mu- should be the preferred state in pure sapphire at low T.