Joseph gives dimensions of the plates and that they had the "appearance" of gold.
Joseph Smith, “Church History,” Times and Seasons 3, no. 9 (1 March 1842): 707
These records were engraven on plates which had the appearance of gold. Each plate was six inches wide and eight inches long and not quite so thick as common tin. They were filled with engravings in Egyptian characters and bound together in a volume, as the leaves of a book with three rings running through the whole. The volume was something near six inches in thickness, a part of which was sealed. The characters on the unsealed part were small, and beautifully engraved. The whole book exhibited many marks of antiquity in its construction and much skill in the art of engraving.