Metal marking appears as a greyish stain on dinnerware and is caused by contact with Stainless steel ( flatware, worktables, sizzle platters, platecovers, dish machine doors, etc.) that either has not been cleaned properly or is of a softer grade SS.
Metal marking is NOT from the dinnerware but rather is caused by a piece of dinnerware having the film from dirty stainless steel transferred to it. Most often this occurs by sliding the plate over an SS work surface, but it also can come from softer SS as used in knives, metal accessory pieces, or from any SS transferring the film. Often an inexperienced dish person will be caught using SS scrubbies to remove stuck on food which will also cause metal marks.
Under most conditions, this washes off in the rinse but on occasion the film remains on the dinnerware and adheres to it when the water evaporates, essentially baking it on. In many cases, it collects in scratches in the glaze, or on the foot of the piece.
In ALL cases metalmarks can be removed by using a product like lemoneze, or dip-it from Ecolab, or by using a mildly abrasive cleaner like soft scrub. Clean the product using a WHITE scrubby and the marks will readily come off.