A pontil is a long iron tube used to collect molten glass for glassblowing, for the blowing of said molten glass, and even for holding the blown article. The Pontil mark is essentially a circular scar left on the bottoms of glass bottles (and other glass items such as marbles, vases, and cups) that were either completely or partially hand blown and molded. The pontil mark was left behind by the pontil, which was attached to the bottom by a blob of molten glass. Once affixed, the bottle could be twirled about, smacked against a handy flat surface, or wielded in a most menacing manner at your young apprentice. The age of the Pontil ended with the American Civil War, and every craftsman back then had a young whippersnapper to fetch heavy objects and run uphill both ways all the while. Haven't you read your history books?
Anyway, once the bottle is finished being used as an object of your wrath, the glass is allowed to solidify and the pontil is broken from the glass item, leaving the circular scar that is held in such high esteem by collectors of such dusty bits of history.