A different, yet equally important use of impurities is as a probe to investigate those physical properties, in a wide range of materials and methods. Impurities are widely used as a means of controlling the physical properties of materials. This establishes that – in analogy with impurity-based methods used in electron-spin-resonance, nuclear-magnetic resonance and Mössbauer spectroscopy – randomly distributed impurities can serve as a non-invasive, but now momentum-dependent probe, greatly extending the applicability of resonant x-ray scattering techniques. Here we show – with simulations and REXS on Mn-substituted Sr 3Ru 2O 7 – that these limitations can be overcome by performing resonant scattering experiments at the absorption edge of a suitably-chosen, dilute impurity. Although the power of REXS has been demonstrated by numerous studies of complex oxides performed in the soft x-ray regime, the cross section and photon wavelength of the material-specific elemental absorption edges ultimately set the limit to the smallest superlattice amplitude and periodicity one can probe. Resonant elastic x-ray scattering (REXS) is an exquisite element-sensitive tool for the study of subtle charge, orbital and spin superlattice orders driven by the valence electrons, which therefore escape detection in conventional x-ray diffraction (XRD).