Some thirty billion tons of gold are concealed within the earth’s crust for an average of three to five milligrams of gold in every 1,000 kilograms of crust. But these few milligrams are far too sparse for efficient and profitable mining. Consequently, over the past 6,000 years humans have recovered and utilized only the relatively minute quantity of 100,000 tons of gold – and only a small fraction of this gold has been used for the production of yellow gold watches, making them all the more valuable. The physical properties of gold have been well known for generations: number 79 on the periodic table; atomic weight of 197; a specific gravity of 19.32 grams per cubic centimeter; and a melting point of 1,063° Celsius. Gold is outstandingly resistant to corrosion and nearly unaffected by acids and oxygen. But pure gold – meaning the 24-karat variety – is too soft to serve as a watch case. Fortunately, gold can be alloyed with other metals, which hardens the material and enables metallurgists to create gold alloys in a wide range of colors, subdivided in categories from 1N to 5N. The typical bar gold or yellow gold has a hardness of 160 to 200 Vickers. So everything is prepared for the production of watches in yellow gold.