This is an antique "Mosaic" green porcelain tile in 4-1/4" x 4-1/4" square. 

The Mosaic Tile Company was incorporated in Zanesville, Ohio in 1894. The company specialized in octagonal, hexagonal, and square floor tiles as well as artistic tile murals, and later, in vitreous and semi-vitreous floor tile. By the 1920's, the Mosaic Tile Co. was one of the largest producers of ceramic tiles in the country, and it acquired the Atlantic Tile Manufacturing Company of Matawan, NJ. The Mosaic Tile Co. had a distinctive glazing style, recognizable by its satin or matte finish, as well as a more artisanal finish, which is not totally opaque like that of other tile companies. This tile, known as "faience," referred to the plastics process that was used to apply the glaze to the ceramic tile. Unable to compete with Japanese and other foreign companies, the Mosaic Tile Co. stopped producing faience tile in the late 1950's and was dissolved in 1967.