Arms which do not comply with the Rule of Tincture.  This rule is followed so greatly that any breach of the rule is thought to be done on purpose, hence the term Arms of Enquiry, meaning that the viewer will automatically wish to inquire why the rule was broken.  The most famous example is the Argent field and Or crosses of the Arms of the Kingdom of Jerusalem, where in the usage of metal on metal was to allude to exceptionally holy nature of the domain.