Dinnerware glazes look and feel very smooth; however, the surface is composed of microscopic peaks and valleys. When metal utensils, which are often softer than the glaze, contact the stoneware, tiny metal particles become caught in the microscopic valleys.
Though some users characterize the gray marks as scratches, usually there is no damage to the glaze or the stoneware body. The metal deposits can be removed with a variety of cleansers, including Bon Ami, Soft Scrub, or Bar Keepers Friend.
As all dinnerware manufacturers will attest, metal marks are not considered a product defect and can easily be removed with a porcelain cleanser.