In the following list, the names of the discoverers and name etymologies of the seven precious metals (again, those with recorded discoveries) are provided. The year of discovery of each of these elements, as presented here, refers to the year when the element was first identified as the pure element. These are, in no particular order, gold, palladium, silver, rhodium, iridium, ruthenium, platinum, osmium, and rhenium. So that while the chemical element tellurium is considered one of the rarest element in the Earth's crust (its mass abundance being the same as that of rhodium, iridium, and ruthenium), its estimated price is only less than 5 U. On the obverse of the Krugerrand is the face of Stephanus Jonannes Paulus Kruger, fifth president of the South African Republic (note the surname Kruger and the South African currency Rand were combined to give this precious metal coin its name). Also here are the Afrikaans and English versions of the name "South Africa", both inscribed in capital letters. Trommel: This method involves the use of a screened cylinder to separate materials by size (trommel is Dutch word for "drum"). A trommel specifically consists of a rotating metal tube that is slightly tilted, with a screen at the discharge end. Attached to the inside part of the metal tube are lifter bars. 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. Because it's wear-resistant and does not tarnish, platinum is highly valued by jewelers, especially watchmakers. In 2008, its price went up to as high as 2,252 U. 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.mol-1 (first), 1260 kJ.mol-1 (second), 2510 kJ.mol-1 (third) Rhenium is silvery-white in appearance. It is the third element (after tungsten and carbon) with the highest melting point and the fourth densest (after platinum, iridium, and osmium).