I stumbled across this video about the world’s largest emerald being sold. Sure it’s being sold by Canadians, but that’s not the important part, the important part is just how a 57,500-carat gem could form in the first place.
I asked a fellow grad student, Tim, who knows about these things, just how an emerald could grow to this size. His answer reads like a recipe, but probably not one you could make yourself.
First, you need a lot of water, heat, and beryllium. Beryllium (Be) is one of the main constituents of an emerald. It’s a relatively rare in continental crust, but can be found concentrated in mafic and ultra-mafic rocks, such as those uplifted from the mantle or oceanic crust. The Be is eventually leached out of the rock by a hot fluid of the right chemical composition, so what kind of fluid does it take?
Here’s where’s the science gets tricky. The water needs to be able to dissolve sufficient amounts of Be, as well as aluminium (Al) and silicon (Si). The full chemical formula for beryl is (Be3Al2(SiO3)6). The green color comes from trace amounts of chromium (Cr) or vanadium (V). According to Tim, the water is usually from a hydrothermal source already containing carbon dioxide (CO2), chlorine (Cl), sodium (Na), calcium (Ca), and magnesium (Mg), so it’s essentially a carbonic acid brine, which are associated with hydrothermal alteration of oceanic crust. These compounds and elements all aid in Be solubility and mobility.
The fluid and heat come from a nearby granitic or magmatic intrusion. Temperatures below around 500 degrees C result crystallize magmatic components, leaving behind the hydrothermal fluid saturated in incompatible elements. The fluid pulls Be out of the oceanic crust, leading to the precipitation of an emerald following subsequent changes in the chemical or physical conditions of the fluid.
That’s how any basic emerald forms, for an emerald this size you’d need a large fluid volume and a particularly high concentration of Be. Very cool! We really don’t talk about hard rock geology on the show enough…
The emerald’s Brazilian name, Teodora, translates to “Gift of God.” I’ll thank geology. And Tim. Thanks, Tim!