In what era did marking jewelry with identification as to amounts
of metals begin?
Well, Jennifer, ancient Mesopotamia strikes again! Actual marks
have not (yet) been found in the archaeological record, but
cuneiform texts from the mid-third millennium (ca. 2400 BC) mention
gold, followed by numbers, which seem to refer to its relative
purity. Describing gold by numbers continued into the Ur III
period (ca. 2000 BC) and later. Gold also was sometimes described
by color, and its price given, relative to silver (which was the
metal most often used in ancient Mesopotamia as a standard of
value). It seems obvious that they knew how to refine gold, but
the texts never describe the process used.
As for copper, texts from the same periods (ca. 2400 BC) mention
specific weights of copper (URUDU) and tin (AN.NA) which were
mixed to form bronze (UD.KA.BAR), and they sometimes even include
the percentage of waste which could be expected in casting.
Whether marks were ever used in the third or second millennia BC is
not known. Perhaps because of ancient recycling and the corroded
condition of most copper and bronze which is excavated, we will
never know. However, ancient Mesopotamian gold artifacts, so far,
show neither maker nor quality marks.
All the best,