Generally buttons did not appear until the 14th century. There are some objects that could be buttons dating to the Iron Age and Roman periods, these look like toggles.
There are 2 types of metal buttons:
Large and flat buttons are generally 17th century or later.
A complete Post Medieval cast copper alloy button. The button has been cast in one piece. The front and the back are both domed. It has an integral loop with circular hole. The button is complete, and is in good condition. The button is 13.2mm long, 9.5mm in diameter and weighs 2.44 grams. Read (2005) page 52, no.182 illustrates a similar button which he suggests dates from the mid 17th to mid-18th centuries.