Description from Rejuvenation
Popular from the Civil War into the 1930s, black doorknobs were called “jet”, and were made from red clay with a black glaze.
White doorknobs were “porcelain”, but were made from white clay with a fired porcelain surface, like china. (“Mineral” was a swirly brown color, which disappeared in the 1920s.)
Until the turn of the century, clay doorknobs were the most common doorknob selection offered on interior door sets.