Green Gold Oh, what a little hot water can do to boring old shale By Robert Kunzig Before the Spanish conquest of what is now Colombia, people in the mountains north of Bogota are said to have thrown emeralds into Lake Guatavita. Once a year the Indian ruler would cover himself with honey and gold dust and at daybreak have his men row him out into the lake. As he plunged into the water, offering the gold to his god, the crowd on shore would throw in their own offerings. The rich ones chucked in emeralds. When the Spaniards finally found the Indian emerald mines after decades of bloody searching, the Old World went crazy for the New World's gems.
Emeralds are valuable because they are rare, rarer than diamonds.
An emerald is a type of beryl, a mineral made of beryllium, aluminum, silicon, and oxygen. All those elements are common in the continental crust, so beryls are not rare. But whereas ordinary beryls are colorless, emeralds are green because a few of the aluminum atoms in their crystal structure have been replaced by atoms of chromium or vanadium. Soon after Earth was born, when it was young and mostly molten, a lot of silicon and aluminum rose to the surface, like a kind of scum, then cooled, forming the first continents. Most of the iron stayed behind in the mantle or sank into the planet's core.