Noble Metals
Among the many elements represented on the Chart of Elements is a group known as the "Noble Metals."  These metals distinguish themselves by being remarkably resistant to chemical reactions, corrosion and oxidation.  Noble metals include gold, silver, copper, platinum, and members of the platinum group:  iridium, osmium, palladium, rhodium and ruthenium.  Noble metals are indivisible; that is, they can occur alone and uncombined with any other element in their native state in nature, and are also known as native elements.  

Noticeably heavy and with a bright yellow metallic color, gold is extremely malleable and ductile (deformed plastically without fracture), with a relatively low melting point (1063 degrees Celsius).  Due to its extreme malleability, one ounce (28 grams) of pure gold can be beaten out to 300 square feet.  Gold is widely used as an international currency standard, and commonly is used for jewelry fabrication and dentistry in part due to its hypoallergenic properties, electronics (due to its superb conduction properties), and in certain photographic processes. 
The allure of gold has been documented since the beginning of recorded history, and at least 6,000 years ago was utilized by the ancient civilizations of Egypt and Mesopotamia.  Probably initially discovered in stream beds as nuggets or flakes, gold also occurs in primary hydrothermal veins, in ancient and more rarely active volcanoes, and also is found in alluvial sources, also called placer deposits.  When gold is found as nuggets, it is rarely found in a completely pure state and often occurs mixed with silver. When the ratio is roughly 75% gold to 25% silver, the material is called Electrum.  Gold also may occur in granitic pegmatites or in contact metamorphic deposits.  Crystallized gold is very rare, but when found make beautiful aesthetic specimens that are increasingly left as is rather than destroyed for their gold value alone.  Forms of gold crystals include rounded octahedra, cubes, and dodecahedra to 2 cm. and often elongated along certain crystallographic planes resulting in herringbone and dendritic twin forms.  Gold also occurs in flattened plates with triangular octahedral faces, and rarely as wires.  Other forms include massive rounded fragments, flattened grains, and as gold dust. 
The word “gold” is derived from the Anglo Saxon word of the same name; in Latin, gold was called aurum, hence the abbreviation “Au” for gold in the periodic table.  Gold is very soft in its pure state, and in order to make it more wearable (as well as extending the amount of gold available), it is usually mixed or alloyed with one or more other metals to add stability and keep the gold from literally wearing away by the action of rubbing and hitting.  Pure gold is termed 24 Karat (Kt.), meaning out of 24 parts, 24 of those parts are gold.  The terms 18 Karat and 14 Karat indicate that 18 parts (75%) or 14 parts (58.3%), respectively, of 24 parts are pure gold.  The other six to ten parts generally consist of silver, copper, zinc, and, in the case of colored gold, other metals.  More copper mixed with gold results in red or pink gold.  Green gold is created by adding more silver.  Other gold colors include black, brown, purple and blue.  Rarely seen is blue gold, obtained by adding iron to gold.  To create white gold, palladium or nickel is added to the recipe, creating a very light yellow gold.  To whiten the gold further, the object is then plated with rhodium, a platinum group metal, resulting in the white gold commonly used in jewelry.  White gold became popular during World War II, when platinum was unobtainable due to its application in military weapons.  Primary sources for gold include South Africa, Russia, Brazil, Australia, Canada and the United States.
Gold Properties
Chemical Symbol:  Au
Crystal System:  Cubic
Specific Gravity:  19.3
Hardness:  2 ½ - 3
Color:  Yellow
Melting Point:  1063 degrees Celsius
Platinum is a member of a suite of similar white or gray colored metals, including platinum, ruthenium, rhodium, osmium, iridium and palladium, and are collectively known as the Platinum Metal Group (PMG).  Platinum has been used for thousands of years, but wasn’t considered a distinct metal until 1735.  The word platinum is derived from the phrase “platina,” with platina being a derogatory Spanish term for “little silver” in the mistaken impression that the metal was an impure ore of silver.  Because of the high melting temperature of platinum, the Spanish did not know how to work with it and did not value it the way they did gold.  Native platinum is almost always found in conjunction with iron or other metals, such as iridium, rhodium and palladium.  Platinum is generally found in nature in the form of small grains, and rarely as nuggets or crystal. 
Platinum gained more interest after the early 1800s, when William Hyde Wollaston and Smithson Tenant discovered a way to refine platinum so that it could be produced commercially.  In the 1890s, the French jeweler, Cartier, found that platinum was the perfect metal to express the garland style of jewelry made popular by him.  Strong and resistant to oxidation or attacks by acids, platinum could be used effectively in the popular “knife-edge” settings of the delicate Edwardian style jewelry fashionable at the turn of the century.  In the Art Deco period of the 1920s and 1930s, platinum overtook gold as the preferred metal for showing off the popular diamond styles of the day, and was only usurped by gold at the beginning of World War II, when platinum was declared a strategic metal and the U.S. government banned its use for anything other than military applications.  Following World War II, platinum gained some of its popularity back when used for the light and airy designs of Harry Winston, where the smallest amount of metal was used to show gemstones more prominently and the settings needed to be strong.  But, generally, until the early 1990s, white gold was more acceptable to the American public because it was less expensive.  In the early 1990s, platinum again became highly fashionable in America due in part to the formation of Platinum Guild USA (PGI); a marketing and information group that offered technical training to jewelers in order to promote the positive aspects of this beautiful metal.  
Today platinum is generally mixed with ruthenium, iridium or palladium to make it less brittle, and to extend the amount of the precious material as much as possible.  Popular alloys of platinum include 950 platinum, composed of 95 parts of platinum to 5 parts of ruthenium, which has become a popular formula for cast and die-struck platinum.  In the 1920s and ‘30s, 90% platinum to 10% iridium was a popular alloy.  Platinum jewelry is generally stamped "PLAT," "PLAT 950," "PLAT 900," "Pt 950" or "Pt 900," or "10% Irid. 90% Plat.," depending upon the alloy being used.
The largest known reserves of platinum occur in the Bushveld complex in South Africa.  Russia, Canada and, to a lesser extent, the U.S. and Colombia also contain significant deposits.          
Platinum Properties
Chemical Symbol:  Pt
Crystal System:  Cubic
Specific Gravity:  21.4
Hardness:  4 – 4 ½
Color:  whitish-Gray
Melting Point:  1,773.5 degrees Celsius
Discovered by Smithson Tenant in 1803, iridium is a native element of the platinum group.  Iridium is white, hard, brittle, and very heavy, and is not affected by acids.  One of the rarest of all elements found in the earth’s crust, iridium is used in combination with platinum to make a harder alloy for use in jewelry and precision instruments.  Iridium is the second densest element known next to osmium.  Iridosmium is a naturally, if rarely, occurring hexagonal alloy of iridium and osmium.  Iridium occurs in nature as a natural alloy of platinum in the Bushveld complex of South Africa, Russia, Canada and, to a lesser extent, in the U.S. and Colombia.
Iridium Properties
Chemical symbol:  Ir
Crystal System:  Cubic
Specific Gravity:  22.42
Hardness:  6 – 6 ½
Color:  White
Melting Point:  2,350 degrees Celsius

Discovered by Smithson Tenant in 1804, osmium is a native platinum group element that is bluish-white in color, very heavy, extremely hard and not attacked by acids.  Remaining extremely lustrous even in high temperatures, osmium is usually alloyed with platinum and iridium for use in pen tips and surgical instruments.  Osmium is the densest of all elements known.  Osmiridium is a natural hexagonal alloy of osmium and iridium. Osmium is rarely found alone in nature, usually binding with platinum and/or iridium.  Occurrences include the Bushveld complex in South Africa, Russia, Canada, and, with lesser deposits, in the U.S. and Colombia.
Osmium Properties
Chemical Symbol:  Os
Crystal System:  Hexagonal
Specific Gravity:  22.48
Hardness:  7
Color:  Bluish-White
Melting Point:  2,700 degrees Celsius
Palladium is another member of the platinum group and generally occurs with platinum ores.  It is the softest metal of the platinum group and also melts at lower temperatures than the other platinum group metals.  Silvery white in color and ductile, it also is the least dense of any of the platinum metals.  Palladium has been used as a precious metal for jewelry since 1939, when, with the onset of World War II, platinum was declared a strategic resource and prohibited for use in jewelry in the United States.  It is often combined with gold to create white gold, and, due to its whiteness, does not need additional rhodium plating.  In 2001, difficulties in casting palladium were overcome and prices started to decrease.  In 2004, the sharp rise of platinum prices spurred much greater demand by manufacturers and showed a marked increase, especially in China.  The use of palladium as a primary metal for jewelry continues to increase today.  Palladium occurs primarily in Russia, South Africa, Canada, and the United States.
Palladium Properties
Chemical Symbol:  Pd
Crystal System:  Cubic
Specific Gravity:  12.02
Hardness:  4.8
Color:  Silvery-White
Melting Point:  1,553 degrees Celsius

Rhodium is a platinum group metal (PGM) that is white in color and highly reflective.  Discovered by William Hyde Wollaston in 1804 soon after he discovered palladium, rhodium has a higher density and lower melting point than platinum.  Rhodium is used in jewelry for electroplating of white gold, making the material more reflective and whiter in appearance.  Although very high in price ($80,000/kilogram in 2010) the chief reason rhodium is not used more frequently in jewelry is due to its poor malleability, making it difficult to fabricate anything with it.  Primary occurrences are in South Africa, Russia, and Canada.

Rhodium Properties
Chemical Symbol:  Rh
Crystal System:  Cubic
Specific Gravity:  12.44
Hardness:  6
Color:  White
Melting Point:  1,985 degrees Celsius
Karl Klaus discovered the white platinum group metal ruthenium in 1844 as a result of experiments with nuclear fission.  Ruthenium helps to strengthen and harden platinum and palladium when used as an alloy, and is sometimes used for electroplating jewelry, as it is cheaper than rhodium and maintains similar properties. Primary occurrences include South Africa, Russia, and Canada.
Ruthenium Properties
Chemical Symbol:  Ru
Crystal System:  Hexagonal
Specific Gravity:  12.06
Hardness:  6.5
Color:  Grayish-White
Melting Point:  2,450 degrees Celsius 
Silver is more widely distributed in nature than gold or platinum, but occurs rarely in respect to other metals.  This relative rarity, in addition to other properties, has made silver an excellent material for coinage; as such, it has been used for monetary and decorative purposes by many different cultures for at least 2700 years.  Silver ornaments have been found in ancient tombs dating back to 4000 B.C.E.  The earliest silver mines are in Turkey, and are estimated to have been worked from 4,000 B.C.E.  Softer than copper and harder than gold, silver has the highest thermal and electrical conductivity of any metal.  Next to gold, silver is the most malleable (beatable or compressible) and ductile (deformed plastically without fracture) of all metals.  All of these qualities combine to produce a relatively hard but elegant white metal that takes a high polish and can be utilized for many industrial applications, as well as for jewelry, hollowware, and flatware.  
Silver jewelry is commonly stamped with a quality mark in the United States.  Sterling or solid silver is stamped “925,” denoting 92½% silver to 7½% copper and/or other base metal for hardening and extending the quantity.  Fine silver is stamped “.999” and is considered pure silver, the other .001 being impurities.  Britannia Silver is stamped .958, but is rarely seen in the United States.  Mexican silver is generally stamped .950; although items stamped between 1930 and 1950 tend to be .980 in fineness.   
Silver Properties
Chemical Symbol:  Ag
Crystal System:  Cubic
Specific Gravity:  10.1 – 11.1
Hardness:  2 ½ - 3
Color:  White
Melting Point:  960.5 degrees Celsius
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