Metal detectors can discriminate between ferrous and non-ferrous easily.
They cannot easily tell apart non ferrous metals, especially those with similar conductivity and similar mass.
Gold and aluminium are similar, likewise copper and silver.
The cheaper metals have slightly lower conductivity so need to be slightly larger mass to give a similar reading to the expensive ones - so eg. reasonable size copper and aluminium washers should give the same results as silver or gold rings.
Having done a bit more technical research since my last answer, sheet aluminium foil would not work well as it will not retain an induced current for long - but metal washers should be ideal.
eg.
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Pieces of copper or aluminium sheet metal would also work to confuse things.
This is a small extract from a very long technical article on metal detector technology:
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As you can see, the detection effects are as much from the mass as from the metal type.