I have finally gotten around to getting the answer to the
question regarding quality stamping of pieces made of silver and
gold. The Jewelers Vigilance Committee, Inc. sent me a copy of
Voluntary Product Standard PS 68-76, "Marking of Articles Made of
Silver in Combination With Gold". I'll try to summarize it.
In short, there are two different categories, articles where the
two metals are visually indistinguishable and those that are
visually (you guessed it) distinguishable.
If you can't readily see the difference between the two metals,
the piece should be stamped "Sterling and" or "Sterling +"
followed by the fraction of gold to the weight of the entire
metal piece and the karat fineness of the gold used. Example:
"Sterling and 1/5 10K". In order to take credit for the gold on
a piece, it must comprise at least 1/20 of the entire weight of
the metal in a piece and must be at least 10K.
On pieces where it is obvious what is gold and what is silver,
the item should be stamped "Sterling and" or Sterling +",
followed by the karat fineness of the gold. Example: "Sterling +
14K". If you want, you can also add the fraction of the gold
weight to the total metal weight.
There was no on tolerances, however, the fractions
do have to have a "1" in the numerator, and the document says
that "...the only fractions recognized by this Standard shall be
1/2, 1/3, 1/4, 1/5, 1/6, 1/7, 1/8, 1/9, 1/10, 1/15, and 1/20."
Hope I made some sense and that this helps.
Enjoy your day.