Platinum - VIIIb/6; Pt -> Pt2+ + 2 e-; 1.18 V 8. Gold - Ib/6; Au -> Au3+ + 3 e-; 1.498 V The second item appearing after the name of the metal (the three items are separated by semi-colons) represents the metal's reaction in water. In the pH diagram, the pH symbol is labeled on the horizontal axis to signify the -log function of the concentration of H+ ion. Three Methods Of Assay For Raw Precious Metals Items of jewelry or art made of precious metals are hallmarked based on specific requirements either of the country of import or the place of manufacture. Such items, particularly those made of silver, gold, or platinum, are struck with an official mark (or series of marks), which guarantees fineness or purity of the metal used. For example, osmiridium (a natural alloy of osmium and iridium) is alloyed with the other metals in the platinum group and used in instrument pivots and phonograph needles (apart from electrical contacts and fountain pen tips as mentioned earlier). In another example, osmium tetroxide is used to detect fingerprints and to stain fatty tissue for optical and electron microscopy. Compared to gold, platinum is more precious, although its price is considered more volatile. One of the reasons for this is that its demand is driven by industrial uses or applications. For example, its price tends to be double that of gold's when world economy is stable, but significantly goes down in times of economic uncertainty. Bullion is traded on commodity markets in two forms: bulk ingots or coins, the latter minted by the government of a country. At least ten countries are known to mint gold and silver bullion coins. These are Australia, Austria, Canada, China, Mexico, Poland, South Africa, Switzerland, the United Kingdom, and the United States. It occurs naturally in its pure form, as an alloy with other metallic elements (especially gold), and in chlorargyrite and other minerals. As one of three coinage metals (the other two being copper and gold), silver is very malleable and ductile. Of the different metals, silver is known to have the highest thermal conductivity.