What are the standard jewelry industry terminology terms to use for items made of gold?

Only 24-karat gold can be properly referred to as “Gold”, without any other modifier, because 99.999% of 24K is the element Gold.  Thus 24K is considered pure Gold.  All other gold colored metals, whether they contain the element Gold or are simply imitations, need have the correct modifiers just before or just after the word gold to define them.  A proper modifier that can be used would be to specify 'karat,' as in "18K gold."

A karat is a measure of fineness. It denotes the proportion of pure Gold to alloy in the finished metal, the higher the karat designation the greater the proportion of Gold to alloy.

For instance, 14 karat gold is properly stamped 14K, indicating the Gold in the article weighs 14/24th of the total weight of the item, with the other 10/24ths being alloys.

Various countries have traditionally used different purities in the manufacture of Gold jewelry. The standard in the US is typically 10K and 14K. In many European countries, 18K gold is the lowest fineness allowed for sale as Gold. In India and the Far East, 22k has often been used. And in the United Kingdom, 9K (or 9ct) gold is the lowest fineness sold as Gold.

Other marks typically used on jewelry can denote that their Gold content is only mechanically bonded to the surface, such as "1/20 12K" which means, 12 Karat Gold Filled, or "14K H.G.E" which indicates 14K Heavy Gold Electroplate.

The vast majority of vintage costume jewelry is simply gold tone metal, with no actual pure Gold content. Although it is tempting to simply call it Gold in the title and description, it should be called Gold Tone or Gold Color to prevent any possible shopper confusion. Remember, Gold can only properly be used to refer to 24K gold.

 What is Rhodium

Rhodium is a silvery-white hard metal often found in platinum ores. It is the most expensive precious metal.  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.

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