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ordinary beryls are colorless, emeralds are green because a few of the aluminum atoms in their crystal structure have been replaced by atoms of chromium or vanadium.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. Other elements chose one of those two fates, too, based on their weight and size.
Geologists call the light minerals felsic and the dark ones mafic.
The paradox of the emeralds, as Cheilletz calls it, is that beryllium belongs to the light, felsic, continental side, whereas chromium and vanadium are from the dark, mafic, oceanic side. Emeralds, in other words, are yin and yang in a single crystal. The answer, they discovered, has to do with plate tectonics, the ceaseless shifting of Earth's crust that smashes continents together to build mountains. Every now and then, when an ocean disappears between two colliding continents, a chain of volcanic islands or a slab of seafloor gets beached on land. To make an emerald, though, those elements have to come together in a single hot liquid.