Fineness

Alloy metals are added to increase hardness and durability of coins and jewelry, alter colors, decrease the cost per weight, or avoid the cost of high-purity refinement.

For example, copper is added to the precious metal silver to make a more durable alloy for use in coins, housewares and jewelry.

[2] Millesimal fineness is a system of denoting the purity of platinum, gold and silver alloys by parts per thousand of pure metal by mass in the alloy.

The carat (UK spelling, symbol c or Ct) or karat (US spelling, symbol k or Kt) [17][18] is a fractional measure of purity for gold alloys, in parts fine per 24 parts whole.

First attested in English in the mid-15th century, the word carat came from Middle French carat, in turn derived either from Italian carato or Medieval Latin carratus.

These were borrowed into Medieval Europe from the Arabic qīrāṭ meaning "fruit of the carob tree", also "weight of 5 grains", (قيراط) and was a unit of mass[21] though it was probably not used to measure gold in classical times.

[22] The Arabic term ultimately originates from the Greek kerátion (κεράτιον) meaning carob seed (literally "small horn")[22][23][24] (diminutive of κέρας – kéras, "horn"[25]).

In 309 AD, Roman Emperor Constantine I began to mint a new gold coin, the solidus, that was 1⁄72 of a libra (Roman pound) of gold[26] equal to a mass of 24 siliquae, where each siliqua (or carat) was 1⁄1728 of a libra.

XRF will measure only the outermost portion of the piece of metal and so may get misled by thick plating.

A refiner doing $1 billion of business each year that marked .980 pure bars as .999 fine would make about an extra $20 million in profit.