After having proved the metal with aqua fortis, which I found in my apothecary shop, likewise with other experiments, and read the long article 'gold' in the Encyclopedia Americana, I declared this to be gold of the finest quality, of at least 23 carats." — From "The Discovery Of Gold In California," by "Captain" John Sutter, California Magazine, November of 1857.
Gold has long held a curious allure for humankind. Egyptians, Sumerians, Aztecs and Incas independently recognized its worth, both symbolically and in real monetary terms, as a medium for exchange. Yet there's nothing intrinsically precious about gold; it's valuable only because we all agree it's valuable. So what is it about this element, atomic number 79, that makes it special? Some unique physical qualities include:
Color. Gold has a lustrous yellow-orange sheen, especially in its pure 24-carat form. (The chemical symbol for gold, Au, comes from the Latin aurum, with the Proto-Indo-European root also responsible for Latin aurora, dawn.) The outer shell electrons of gold move at relativistic speed — 58 percent the velocity of light — resulting in absorption of the low blue wavelength of white light. Blue's complement, yellow, is reflected back to our eyes. Scientists only figured this out decades after Einstein's general relativity, using a new branch of science, relativistic quantum chemistry, which combines chemistry and physics.
Ductility. Gold is the most ductile of all metals. Consider a single ounce of gold such as a krugerrand or a U.S. $50 Gold Eagle coin. According to the American Museum of Natural History, this ounce can be stretched into a 50-mile length of wire — Fortuna to Garberville, say — at a thickness of just 5 microns! (Human hair is about 75 microns across.) Alternatively, you could beat that ounce into a super-thin square of metal measuring 10 feet on the side. Light shining through it would appear blue-green.
Corrosion Resistance. Gold has the highest corrosion resistance of all metals. It won't oxidize (in common with other "noble metals") and, unlike silver and base metals, doesn't dissolve in nitric acid, hence the term, "acid test." In January 1848, Sutter just happened (!) to have a bottle of nitric acid (aqua fortis) handy when his partner, James Marshall, brought him some scraps of shiny metal he'd found in the tail race of Sutter's sawmill — see quote above. Incidentally, fool's gold, or iron pyrite, looks similar to gold and won't dissolve in nitric acid, but it's brittle and it oxidizes, unlike the real thing.
Historically, gold is unequalled as a medium of exchange. Consider the options for early civilizations, when traders began to realize the advantages of money over a straight "my three sheep for your ox" barter system. Perhaps they considered other metals like iron, copper, lead, silver, palladium and platinum. The first three corrode easily and silver tarnishes, while the noble metals palladium and platinum are too rare. Gold is ideal for coinage, being somewhat rare and difficult to extract, yet sufficiently abundant to be minted for commerce. How abundant? One estimate for the total amount of mined gold (mostly from South Africa, China and Australia) is 170,000 metric tons, equivalent to a solid cube with 60-foot edges.
So gold has a curious split personality. It's worth plenty (nearly $2,000 an ounce currently) but it also appeals to something deep-rooted and intangible beyond monetary value. Think of Olympic medals, Oscar statuettes, wedding bands and other fine jewelry, 5,000-year-old gold chains from Ur, Agamemnon's death mask and Incan sun discs. Want some? You're not alone. As Gerald Loeb, founder of the E.F. Hutton brokerage company, put it, "The desire for gold is the most universal and deeply rooted commercial instinct of the human race."