In other instances, gold nuggets are found in piles of residue in sites where mining operations once took place. Two gold nuggets are noted for being the largest masses of gold ever discovered. These are the "Welcome Stranger" and the "Hand of Faith". Their respective "largest" titles, however, carry further qualifications. In earlier times, precious metals were mainly used as currency. Today, however, they are highly regarded as investment and industrial commodities. Four of them, in fact - silver, gold, platinum, and palladium -, are minted into coins or cast into ingots and traded on commodity markets. All four are assigned the ISO 4217 currency code. Known to be highly resistant to corrosion, this precious metal is used in jewelry, electrical contacts, and laboratory equipment. Its price, as of January 2010, is USD1,555 per troy ounce (USD49,995 per kilogram), making it the second most expensive precious metal, after rhodium. Evident from the description of each of these six precious metals is that all of them have outstanding catalytic properties and high resistance to tarnish and wear. The name rhodium was derived from the Greek word "rhodon", which means "rose". 6. Ruthenium - Karl Karlovich Klaus, a Russian chemist and naturalist, is usually credited with the discovery of ruthenium (in 1844). He discovered it from platinum ore samples he obtained from the Ural Mountains in Russia. mol-1 Electron Configuration: [Xe] 4f14 5d5 6s2 Physical: Density (near room temperature): 21.02 g.cm-3 Liquid Density (at melting point): 18.9 g.cm-3 Melting Point: 3186 C, 5767 F, 3459 K Boiling Point: 5596 C, 10105 F, 5869 K Heat of Fusion: 60.43 kJ.mol-1 Heat of Vaporization: 704 kJ.mol-1 Atomic: Oxidation States: 7, 6, 5, 4, 3, 2, 1, 0, -1 Electronegativity: 1.9 (Pauling scale) Atomic Radius: 137 picometre Covalent Radius: 151 7 picometre Ionization Energies: 760 kJ. The term "placer gold", therefore, refers to gold that has formed in rocks moved and placed on stream beds by some geological forces and by the action of water. Lode gold tends to erode from its source, distributing itself naturally among other rocks that have been subjected to similar geological forces.