In costume jewelry, all that glitters is never purely gold. Instead, the base metal of rings, necklaces, brooches, and bracelets is plated, washed, or flashed with gold of a fineness typically not greater than 10-karat. When gold is plated onto sterling silver, the result is known as vermeil.

Substituting less-valuable materials for gold, or even watering down pure gold into a cheaper and more marketable form of the precious metal, goes back at least to the 18th century. That’s when, in 1720, Christopher Pinchbeck combined copper and zinc to create a gold-like alloy. Pinchbeck was used by pocket-watch makers and fine jewelers alike.

Mechanical and electroplating techniques arrived with the industrialization of the 1800s. One of the most popular mechanical processes produced a metal that even today is called ...gold filled, even though the gold the customer sees resides on the outside of the piece—”filled” is a persistent misnomer. During the Victorian Era, gold-filled jewelry was especially popular.