I see what you mean by refined now. Yet oxide does not equal rust. Rust is just a common term and is misleading.
My bet is on magnetite Fe3O4 (iron (II,III) oxide).
Quote:
Conclusions
[...]Magnetite proved the best absorber of electromagnetic waves in the single frequency that was tested. Hematite* had some microwave absorbing properties but not as good as magnetite.
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Synthesis and Characterisation of Radar Absorbing Nanocomposites
http://www.met.kth.se/utbildning/4H1...dar%202006.pdf
*Hematite is the red iron oxide.