This brooch is from the Vendel period in Swedish history, which precedes the Viking age. Abstractly designed small brooches like these were fashionable until Viking oval brooches replaced them.
From island of Gotland.; Alastair Bradley Martin, Glen Head, New York (until 1953)
Brown, Katharine R., Dafydd Kidd, and Charles T. Little, ed. From Attila to Charlemagne: Arts of the Early Medieval Period in the Metropolitan Museum of Art. New York and New Haven: The Metropolitan Museum of Art, 2000. p. 310, 358, fig. 25.2.