No Diggity (But Lots of Doubt) Coffee Mug
Thick enamel-coated steel. They are heavy (for enamelware mugs-- still lighter than ceramic), smooth, wonderful to drink from, and crazy sturdy. You can abuse the heck out of them.
Mugs may have small flaws and variations in the enamel coating which are intended to add to individual character and vintage patina of your mug. Be prepared for some small imperfections.