This article is designed to recover and reconstruct the antiquarian, collecting, and documentary activities of Italian artist Hieronymus Odam (c. 1681-1740) in respect of engraved gems. Odam is primarily recognized as a painter, while his contribution to the development of antiquarianism and collecting of antiquities remains virtually unknown. Odam’s speciality was intaglios and cameos. The recently discovered drawings of gems in the Princes Czartoryski Museum in Kraków, the Kunstbibliothek in Berlin, and the Biblioteca Apostolica Vaticana, as well as archival sources—Odam’s correspondence and two dactyliothecae (those of Pier Leone Ghezzi and James Tassie)—prove that he possessed a considerable number of engraved gems and was one of the experts in this particular field. But above all, Odam documented thousands of engraved gems, notably for Philipp von Stosch, in a very accurate and innovative way, straying from the traditional antiquarian approach. The co-operation between Odam and Stosch resulted in illustrations that were designed to show techniques of engraving and styles of the ancient masters, qualities that had not previously been addressed by antiquarians. In Odam’s and Stosch’s approach gems were treated as sources of evidence rather than as images useful for illustrating passages from ancient literature. Odam is a fine example illustrating the transformation of antiquarianism in the first half of the 18th century.