This is a clear Unmarked Laboratory Ground Glass Stopper (also known as a ground glass joint), which is commonly used with laboratory glassware mainly because of the nonreactivity of its borosilicate glass. This stopper has a 3" in diameter horizontal top with a 1.625" long partially frosted hollow glass stem (1.75" in diameter at the bottom). The center of the top is slightly raised and the bottom is marked by an uneven mound of glass, showing that the piece was hand blown by a scientific glassblower. There are also small air bubbles caught in the glass. Maker and age are unknown. This Unmarked Laboratory Ground Glass Stopper is in very good condition.