If you are referring to a quality mark like "22K " or
"sterling" then in the US there is no legal way to indicate the
metals in the piece. The Stamping Act and the FTC Guides just never
took into consideration a multi-metal piece.
Actually, The FTC guides do explicitly take multi-metal pieces into
consideration, which is why the Guide 23.9 states
(a) Deception as to applicability of marks.
(1) If a quality mark on an industry product is applicable to only
part of the product, the part of the product to which it is
applicable (or inapplicable) should be disclosed when, absent such
disclosure, the location of the mark misrepresents the product or
part's true composition.
(2) If a quality mark is applicable to only part of an industry
product, but not another part which is of similar surface appearance,
each quality mark should be closely accompanied by an identification
of the part or parts to which the mark is applicable.
Dos Manos Jewelry