The serene beauty of an emerald’s green depths gives no hint to the casual observer of the extreme conditions where it formed.
Only on closer inspection can you see inclusions that are the tell-tale signs of ancient violence:
- Little fissures and minicrystals
- Hollow cavities where air or bubbles got trapped as the crystal slowly grew out of hot fluids deep in some underground boiler
- Tiny clouds of hydrate those fluids left behind
If you’re lucky, you might even find matter there in its triple-phase state – liquid, solid and gas.
Emerald is the dark green variety of the mineral beryl, a combination of beryllium, aluminum and silicate that has had some of its aluminum ions replaced by chromium or vanadium and iron.
Chromium and vanadium occur in dark igneous rocks with a high magnesium and iron content. These come from our planet’s upper mantle, reaching the surface through volcanism, especially at mid-ocean ridges. The sea floor is made of this mafic type of rock.
Beryl is found in felsic rocks that have high amounts of lighter elements, especially silicon and potassium. Continents are mostly felsic.
Your granite countertop comes from continental crust. It seems very heavy to us, but its specific density is actually much less than basalt or gabbro. On the global scale, felsic rocks float and mafic rocks sink.
So…how do beryl and chromium and vanadium get together to form emerald?
How emeralds are formed
Plate tectonics shifts rock formations all over the place. Well, that’s too much of an oversimplification, but it is the reason why ocean crust sometimes ends up scrunched together with continental rock.
Plate tectonic processes change rock through metamorphism, which involves heat, pressure and fluids.
Beryl, meet chromium, vanadium and iron.
In metamorphic rock on each of the seven continents except Antarctica (where a permanent ice cap hinders geologic exploration), emeralds have been found where a granite pegmatite intruded through mafic rock.
What has happened in these places is that molten felsic rock worked its way into a mafic rock formation, forming an intrusion that geologists call a pegmatite. While doing so, it got near ground water, which it superheated or vaporized. The resulting acidic “soup” of magma and watery fluids then slowly cooled and crystallized, forming emeralds.
Here it is in slow motion.
Crystals form when matter changes phase, for example, when it goes from a gas or a liquid state into solid form.
In our homes, water changes phase when it crystallizes into ice on a window during winter. At that point the air contains more water vapor than it can hold and conditions are right to trigger the phase transition into ice.
In the pegmatite, there is a hot felsic magma in contact with relatively cold mafic rock. At the edges, watery fluids are also circulating. This acid fluid dissolves materials in the rock and circulates them through the outer edges of the cooling pegmatite, where crystals are forming.
Beryllium, aluminum, silicon and oxygen crystallize out of the felsic magma to form the mineral beryl.
Emeralds happen when enough chromium, vanadium and iron have dissolved out of mafic rocks to give those beryl crystals a deep rich green color.
Natural emerald formation is a wonderful but messy process. If your emerald doesn’t have inclusions, therefore, it’s quite possibly man made. (Note: The presence of inclusions doesn’t guarantee that it’s a natural stone – they can be added synthetically.)
Carroll Chatham grew the first synthetic emerald in 1935, almost three decades after high-volume production of artificial rubies and sapphires began.
Synthetic emerald gems are beautiful but more expensive than other synthetic stones because they are much harder to make in the lab, requiring the same sort of hydrothermal solution or flux that is present in nature. The emeralds also grow very slowly, and not all crystals are of gem quality.
The delicate beauty of emeralds hides the violent past in which they formed. We can thank plate tectonics and related volcanic and metamorphic processes that have happened over geologic time spans for this combination of the rare chemical elements of beryllium and chromium or vanadium into one of the most precious of gemstones.
Front page photo credit:
Photograph, by Chip Clark, of the Hooker emerald brooch (G7719). Smithsonian Institution.
Emerald. The International Colored Gemstone Association.
Emerald. Gemological Institute of America.
A version of this article appeared at Helium on August 5, 2011.