No article having a gold alloy

No article having a gold alloy portion of less than
one-twentieth by weight may be marked “gold-filled,”
but articles may be marked “rolled gold plate” provided
the proportional fraction and fineness designations
are also shown. Electroplated jewelry items carrying
at least 7 millionths of an inch (0.18 micrometers) of
gold on significant surfaces may be labeled “electroplate.”
Plated thicknesses less than this may be
marked “gold flashed” or “gold washed.”
Gold is relatively scarce in the earth, but it occurs in
many different kinds of rocks and in many different
geological environments. Though scarce, gold is concentrated
by geologic processes to form commercial
deposits of two principal types: lode (primary)
deposits and placer (secondary) deposits.
Lode deposits are the targets for the “hardrock”
prospector seeking gold at the site of its deposition
from mineralizing solutions. Geologists have proposed
various hypotheses to explain the source of
solutions from which mineral constituents are precipitated
in lode deposits.