Answer: Although the jewelry industry terms "karat" and "carat" are often confused, they refer to two distinct concepts. The former is a unit of proportion that applies strictly to the purity of gold while the latter is a unit of mass that encompasses a number of different gemstones.

More Info: The word karat is derived from the Arab word "qirat", which means 1/24th. As a result, the measurement scale for the purity of gold, or the proportion of gold within any given item to that of the other material(s) present, is calibrated on a scale of one to 24 parts. For example, 24-karat gold is 100% gold; 18-karat gold is 75% gold and 25% other material(s); and the common measurement of 14-karat gold is 58.3% gold and 41.7% other material(s). One reason for the preponderance of 14-karat gold items is that this the lowest threshold of gold purity at which items are resistant to cracking.  

In 2008, University of Arizona anthropology professor Mark Aldenderfer together with an excavation team uncovered the oldest known gold artifacts yet found, in the Peruvian Andes of South America, near Lake Titicaca. Found at the drainage basin of Jiskairumoko was a necklace of turquoise and native gold hammered into shape by a population beginning to settle there around 2155 B.C.

More recently, the U.S. Mint's 2005 issue of American Buffalo Gold Bullion Coins was the first time the federal government .9999 fine 24-karat gold coins. The coins were authorized by the so-called Presidential $1 Coin Act on December 12th of that year and were inspired by the same design of American sculptor James Earle Fraser that were featured on the 1913-minted U.S. nickel.