I have this mug....

The loss of a glass bottom or a severe crack  is a most common occurrence!  Unless the stein / tankard  is named to someone real important throw the damn thing away! It will not be worth the expense to fix it. Even finding someone that can cut a correct sized circle of glass now-a-days is almost impossible.

  http://www.steveonsteins.com/glass-bottom-steins-and-mugs-a-discussion-draft