Rhodium

Rhodium is a silvery-white hard metal often found in platinum ores. It is the most expensive precious metal. Rhodium costs about six times as much as gold by weight. It is never found in mineral form, only being found in trace amounts within platinum or nickel ores. 80% of the world's rhodium comes from South Africa, and world production of the metal is only about 20 tons per year.

 In its natural solid state Rhodium is far too hard to work, but it is used as an alloy in the smelting of platinum and palladium.

Rhodium was discovered in 1803 by William Hyde Wollaston, who isolated it from platinum. He named it after the Latin rhodon, meaning rose. Wallaston was also the discoverer of palladium.

In the 1930's some silverware makers began to use rhodium electro-plating to produce sterling flatware that would not require frequent polishing. The use of rhodium plating spread to white gold and sterling silver jewelry as a means of creating a highly durable, tarnish resistant surface.

The use of rhodium plating on base metal, "pot metal" and stainless steel jewelry  findings and settings did not come into wide use until after World War II.  Advances in technology eventually allowed even thinner layers of rhodium to be bonded with the surface of metals.

While no vintage costume jewelry is "made of rhodium" or "set in rhodium", costume jewelry of good quality was often rhodium plated.