Bright nickel plating might be a culprit
Nickel is a common intermediate layer under rhodium and other
plating. And nickel plating can be magnetic - indeed it has to be
avoided on components in some electronic applications. But whether a
thin, intermediate nickel layer is ever enough to cause the reported
magnetic susceptibility of some silver jewellery is still unclear.
However, we are talking about ‘fake rhodium’. So far I have not been
able to reliably confirm use of chromium plating as an alternative to
rhodium (though rumours abound), but nickel plating is being used on
some less expensive sterling silver pieces (as well as base-metal
costume jewellery and findings).
Nickel plating gives a bright hard finish that has been described as
looking like stainless steel (ie much like rhodium ). There are
electroless application methods available for nickel that give
excellent hard and uniform layers over complex shapes. The
electroless nickel plating can have a range of phosphorus contents -
low phosphorus content nickel platings provides the best
wear-resistance and are magnetic. But, again, is a relatively thin
surface deposit enough to cause the magnetic effect?
I would imagine it is now illegal to sell nickel plated jewellery in
Europe, but even in the more nickel-tolerant US surely it would be
wrong not to declare the jewellery as containing nickel?