PRECIOUS METALS
(GOLD, SILVER, PLATINUM GROUP)

    All precious metal weights are given in troy ounce (oz t); there are 12 troy ounces per pound (lb. t) in contrast to 16 "regular" (avoirdupois) ounces (oz) per pound (lb.).

Gold (Au)

  • Purity or "fineness" denoted by karatage (K); 24 karat gold is pure gold.
  • 12 karat gold is 50% gold and 50% alloyed metal.
  • Gold will alloy with nearly all base metals (Cu, Pb, Ni, etc.); different colors of gold are produced by alloying with different base metals. Common alloys are Ni and Cu.
  • Specific Gravity for pure gold is 19.3.
  • Gold purity can be tested with inexpensive devices that measure electrical conduction, or by the older method of gauging the extent of reaction with dilute acids. Pure gold is inert with respect to dilute HCl and HNO3; most alloys will react slowly in these acids. Gold plating or fill can be detected by testing a scratch in the item.
  • F.T.C. Guidelines governing sales of gold and gold jewelry in this country state the following:
    • 1) Karatage has to be stamped on, with trademark.
    • 2) Terms "pure gold" and "solid gold" can only be used for 24 karat gold.
    • 3) Term "gold" can only be used if gold content is 10 K or better.
    • 4) Term "gold filled" applies only to items composed of a layer of gold pressed (not alloyed) onto a base metal where the weight of the gold comprises at least 1/20th of the total weight of the item. Fineness must be shown by stamp, e.g. "1/20 12 k G.F." means the gold layer is 12 karat gold and comprises 1/20th of the total weight of the item.
    • 5) Term "rolled gold plate" is the same, but gold comprises less than 1/20 of total weight; e.g. stamped "1/30 12 k G.P." means 1/30 of the metal weight is from 12 karat gold plate.
    • 6) "Gold electroplate" or "gold flash" or "gold washed" mean a gold coating that is 10 k or better gold at least 0.000007 inches thick. A variant, "heavy gold electroplate" (H.G.E.P.) is at least 0.0001 inches thick.
  • Pricing and Valuation

Silver (Ag)

  • Pure silver has a specific gravity of 10.5
  • "Fine Silver" is 99.9% Ag
  • "Britannia Silver" is 95.84% Ag
  • "Mexican Silver" is 95% Ag + 5% Cu
  • "Sterling Silver" is at least 92.5% Ag; harder than pure Ag.
  • U.S "Coin Silver" is 90% Ag
  • Vermeil (Vur-MAY) is sterling silver with gold plating.
  • Crude fineness testing is done with dilute nitric acid. Silver + dilute HNO3 yields a creamy white to brown liquid. Alloys containing Cu, Ni, and brass effervesce and yield a green, yellowish green or bluish green color. More precise technique involves gauging reaction and resulting color with Schwerter solution (HNO3 + KCr2 + H2O).
  • Pricing and Valuation

Platinum Group metals (Pt, Ir, Pd, Ru, Rh, Os)

    Group of precious metals (platinum, iridium, palladium, ruthenium, rhodium, and osmium) that share similar chemical and metallurgical properties.

  • Most "platinum" used in jewelry is composed of 90% platinum and 10% iridium, which makes it much harder than pure platinum.
  • 5% ruthenium serves same purpose.
  • Much like gold, platinum purity and alloys are commonly stamped, in abbreviated fashion, on items.  Unlike gold, purity is given relative to 1000 parts (permil), e.g. "950Plat" is 95% pure platinum.
  • Palladium and small amount of ruthenium used in setting heads for platinum jewelry; lighter, less expensive.
  • Rhodium is used as a plating metal. Gives white gold and platinum a hard, bright finish and slows tarnish on stainless steel.
  • Platinum has a specific gravity of 21.45.

Notes Index | Corundum | Beryl | Diamond | Pearl | Opal | Jade | Topaz | Tourmaline
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1998
Updated 08/20/09
Comments and questions to helper@mail.utexas.edu
Department of Geological Sciences
The University of Texas at Austin