GO 340 Gemstones & Gemology
ES 567 Gemstones of the World
Dr. Susan Ward Aber, Geologist & Gemologist
Emporia State University
Emporia, Kansas USA

http://academic.emporia.edu/abersusa/go340/gold.htm

Precious Metals

The precious metals are gold, platinum, and silver. They are all rare, with gold present in 3.5 parts per billion of the Earth's crust, platinum about 45 parts per billion, and silver in 73 parts per billion. These metals are strong and heavy because of their close-packed atomic structures and metallic bonding. Gold's specific gravity is 15-19.3 (density of 11 ounces per cubic inch). Silver's specific gravity is 10.5 (6.25 ounces per cubic inch). Platinum's specific gravity is 14-19. Osmium, a sister metal of platinum, is the heaviest metal on Earth with one cubic foot weighing three quarters of a ton! (St. John, 1984, p. 126).

Gold

Silver

Platinum

Precious Metal Facts


GOLD

Gold is found on Earth and an equal amount is present in sea water, but very expensive to extract. It does not rust or corrode and combines beauty with easy workability, rarity, and virtual indestructibility to be a fascination to civilizations throughout time. Goldsmiths and tools were few in ancient civilizations and they worked only for the royalty and very wealthy. Today goldsmiths have machines as well as hand techniques to work the metal and make it possible for just about everyone to own and wear karat gold jewelry.

"Eighty percent of world gold production goes to gold jewelry" (JCK online, May 10, 2004). Peru is still the 6th largest gold producing country in the world according to a 2010 article in Engineering and Mining Journal (http://www.e-mj.com/index.php/features/877-perus-large-precious-metals-and-copper-producers). Gold is soft or can be scratched easily, and it has a malleable and ductile tenacity. Gold has a hardness of 2.5-3 and one troy ounce of gold (around the size of a sugar cube and equal to about 31 grams) can be hammered into a sheet (called gold leaf) covering 108 square feet or pulled into a thread fifty miles long. Gold in a pure state is too soft for use in jewelry. Because of gold's softness and malleable tenacity, it is mixed or alloyed with other metals to make it strong and keep it from bending too readily. Prongs of 20-24K gold are usually not strong enough to hold the gem securely, unless it is "gold 990" or an alloy of gold (90%) and titanium (10%) (Matlins and Bonanno, 1998, p. 206).


Monkeys support the stairway to
heaven on the grounds of the
Grand Palace, Bangkok, Thailand.
Photo by S.W. Aber, 9/2008.
Grand Palace, Bangkok, Thailand.
Photo by S.W. Aber, 9/2008.

Karat is a measurement of fineness or purity of gold and abbreviated "K" or "KT." Pure or fine gold is 24 karat or by the European system, 1000 points fineness. If the gold is 24 karat, 24 parts out of the total are gold. If gold is 18 karat, 18 parts of the whole are gold and 6 parts are some other metal. In other words, 18 karat is 75% gold (18/24 = 3/4).

Gold Measure
Karatage Gold Content Fineness
24K 100%1000
22K 92% 916
20K 83% 833
18K 75% 750
14K 58% 585
12K 50% 500
10K 42% 417
9K 38% 375

Many countries have a minimum karat standard for an item to be called gold. The minimum legal standard of karatage in the U.S. is 10K. Anything less than 10K can not be called gold. The minimum standard for Canada and England is 9K, Mexico is 8K, and in Italy and France, 18K. In the U.S., gold articles do not have to carry a karat or other quality mark. However, if a karat mark is applied, it must be registered (manufacturer's registered trademark stamped near the karat mark) and accurate in accordance federal law.

An alloy, or mixture of metals, is not only to produce optimum strength, but also color. Various metals are mixed with gold to produce stronger jewelry and different colors. White gold is often plated with rhodium, a sister metal to platinum, to give it a whiter finish.

Gold Colors
Gold Color Elements Alloyed with Gold
Yellow Gold-Au, Copper-Cu, Silver-Ag
White Gold-Au, Nickel-Ni, Palladium-Pd, Platinum-Pt, Zinc-Zn
Green Gold-Au, Silver-Ag, Copper-Cu, Zinc-Zn
Pink Gold-Au, Copper-Cu

Karat Gold Jewelry Techniques

  • Hand fabrication is used when detailed designs make it impossible to mass produce the piece. Gold shot, or gold metal grains of any karatage, is used to create gold in sheets or wire that is pulled through a draw plate. The design is created on paper, cut out and glued onto gold sheet. A jeweler's saw is used to cut out the patterned pieces. The design is pierced or cut into the shape, bent and hammered, and finally soldered with karatage melt. The soldering process bonds metal to metal by the use of heat and a filler metal called solder. Filing, sanding, and polishing complete the process. As a final step, the karatage of the metal is stamped into the piece and the jewelers personal goldmark logo to guarantee quality. Precious metal markings and hallmarks can show where the piece was made and how old it is, by identifying the guild, town, country, or company.

  • Lost wax casting has been practiced for some 6000 years and the method lends itself to mass production today. A model of the finished product is carved in wax and positioned in a container. The "investment" is poured in, surrounding the design. The mold is heated, which melts the wax and creates a perfect mold or hollow form of the original design. The mold is then filled with gold or silver, creating a perfect cast of the design. Lost wax casting will be the subject of the next lecture.
  • Die striking produces a stronger product than casting the metal and is often used for prong settings of a ring. Gold is malleable and the design can be stamped, cut, or formed with the proper machinery. Die struck and cast items can be soldered together for a variety of combinations.
  • Gold electroforming could be equated to a paper-mache ball created by wrapping strips of plastered paper around an inflated balloon. When the paper hardens the balloon is popped, leaving a hollow sculpture. Electroforming uses gold particles electrostatically applied to a wax or silver form (called a mandrel) that is later removed. It is a process that allows large but lightweight gold jewelry; it is hollow with seamless surfaces virtually impossible to do in standard manufacturing processes. In theory lightweight gold electroforming should mean lower prices, but until recently the process was only done by Israeli & European manufacturers. Now, technology and new equipment has brought electroforming to the mass market and the U.S. is playing a leading role.

    Non-Solid Gold Jewelry Techniques

    Even though pure gold is alloyed with other metals, which is measured as karated gold, it is solid gold according to the FTC ruling from 1967. In other words, the metal is a mixture of varying amounts of gold, but it is still a solid mixture throughout. The antithesis of this is a gold imitation or the process of encasing a base metal in gold. The former is not gold and the latter, not solid gold but rather a thin veneer of karated gold. The most common gold imitation is a mixture or alloy of zinc, copper, and tin that create a yellow colored metal. Metal can also be given a gold wash or finish. The base metal is brass or steel and the product is electroplated with a non-standardized thickness of gold. Encasing a particular base metal in gold goes by several names that reflect the application method of the gold overlay, plating, or elecrolytic process.

    Gold Facts and Fun

    For statistics and general information on gold, visit these external webpages: