Gold is one of the most precious metals in the world. It is present in the rivers, seas, and the earth's crust and trace amounts are present in plants and animals. It is, however, difficult and expensive to extract. In modern mining operations approximately 3 tons of ore are needed to extract one ounce of gold. The many desirable qualities found in gold, along with its scarcity, have made it the most popular metal for use in jewelry today.

Properties of gold in its pure state

• Has a melting point of 1945 degrees Farenheit (1063 degrees Celsius). When alloyed (chemically combined) with other base metals the melting temperature of the resulting alloy is changed. 18K yellow gold has a melting point of 1675 degrees Farenheit and 14K yellow gold has a melting point of about 1550 degrees Farenheit.
• Has a specific gravity of 19.33. It is relatively heavy compared to most metals, such as silver (SG 10.7) or iron (SG 7.8). A notable exception is platinum (SG 21.4).
• Is more malleable than any other metal and can be hammered into foil so thin that it is almost transparent.
• Has a unique ductility property allowing it to be drawn into wire so fine it can barely be seen.
• Is deep yellow in color. Its great reflectivity properties help keep its brightness and color from fading with time.
• Will not rust, tarnish or corrode. Gold jewelry recovered from ancient Egyptian tombs is in the same state as when placed there over 4000 years ago.
• Is softer than most other metals. On the Mohs scale of hardness (which is a measure of a gemstone or mineral's resistance to scratching), gold has a hardness value of 2 to 2.5. Diamond has a value of 10. Pure gold may easily be scratched. Fortunately, gold becomes harder when alloyed with other base metals.
• Is relatively scarce and therefore expensive. It is estimated that only 125,000 tons of gold have been mined the world over since the beginning of time.
• Is able to bond with other base metals. This property gives rise to the many different colors available in modern gold alloys.

Fineness of gold (karat value)

Since ancient times the purity of gold has been defined by the term karat, which is 1/24 part of pure gold by weight. Pure gold is equivalent to 24K. Gold purity may also be described by its fineness, which is the amount of pure gold in parts per 1000. For example, a gold ring containing 583 fine gold has 583 parts (58.3%) gold and 417 parts (41.7%) of other base metals.

Federal Trade Commission rules require that all jewelry items sold in the United States as gold shall be described by "a correct designation of the karat fineness of the alloy." No jewelry item less than 10K may be sold in the United States as gold jewelry.

The following table lists the relationship between different international gold markings.