Grindstone Island is the fourth largest of the Thousand Islands in the St. Lawrence River and the second largest of the American islands in the St. Lawrence. The island lies near Lake Ontario and is part of the United States of America. In particular, the island is part of the Municipality of the Town of Clayton in Jefferson County, New York. The island consists of Potters Beach, a private vineyard owned by the Purcell family, summer gift shops, four active beef farms, historic but now abandoned farmlands, and private non-government conservation lands (TILT). The current number of year round households who call the island home is ten. The population climbs to 700 people during the summer months. New York State's last operating one-room school was on the island and closed in 1989.