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.
♦ Emeralds
Emerald is the mineral beryl, a combination of beryllium, aluminum and silicate, that has had some of its aluminum ions replaced by chromium or vanadium, giving it the green color. Sometimes, as with the darkest Colombian emeralds, there will also be a little bit of iron to add in just a shade of blue.
Chromium and vanadium occur in mafic formations of dark igneous rocks with a high magnesium and iron content, like basalt and gabbro, that come from our planet’s upper mantle and reach the surface at mid-ocean spreading ridges. The sea floor is made of mafic rock.
Beryllium is found in felsic rocks. Continents are mostly felsic, made of light-colored igneous formations with a high silicate (oxygen and silicon) content. We usually think of continental granite as being very heavy, but it’s much less dense than basalt or gabbro and it will float where mafic rocks will sink.
♦ How emeralds are formed
Molten granite, at the top of an underground collection of felsic magma, will work its way into any weak points found in the overlying rock, forming an intrusion that geologists call a pegmatite. While doing so, it sometimes gets near ground water, which it superheats or vaporizes. When that happens, beryllium is one of the elements that can combine and crystallize out of this hot, acidic “soup” as it cools down over thousands of years.
Crystals form when matter changes phase, that is, when it goes from a gas or a liquid state into solid form. At Earth’s surface, water does this 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 of water vapor into ice. Deep underground, under intense pressure and heat, minerals undergo phase changes, too. Beryllium, aluminum and silicate (silicon and oxyen) can crystallize into the mineral beryl.
With beryl deposits located up in the continental rocks, and chromium/vanadium down at the bottom of the sea, the chances for an emerald forming would be slim, if not for the restlessness of the Earth’s surface.
The tectonic plates that make up the planet’s outer crust are constantly in motion, shifting rock formations all over the place and bringing about collisions where mountains may rise or the sea floor may subduct under a continental edge, softening as the heat increases down through the miles until it ultimately melts as it approaches the hot outer mantle. Plate tectonics is the ultimate source of the intense pressure that, in addition to heat, changes the internal structure of rock formations and individual minerals over time. Metamorphosis is the technical name for this process.
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 intrudes through mafic rock, introducing some chromium or vanadium into its growing beryl crystals. Brazilian emeralds, for example, which make up some 10% of the world’s emerald production, come from pegmatites.
However, it’s the presence of hot water solution that counts, not the granite itself. In a few cases, other tectonic processes can send extremely hot water through continental and sea bed rocks, dissolving all the necessary ingredients to make an emerald. This is the case in Colombia, which leads the world in the quantity and quality of its emeralds although there isn’t any sign of magma intrusion in those regions. Experts believe that circulation processes in the local black shales, as well as regional metamorphism and tectonic processes like shear zone formation, formed Colombian emeralds.
♦ Synthetic emeralds
Carroll Chatham grew the first synthetic emerald in 1935, almost three decades after high-volume production of artificial rubies and sapphires had begun. Synthetic emerald gems are beautiful and difficult to distinguish from natural ones, but they are more expensive than other man-made stones because they are much more difficult 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 combining the rare chemical elements of beryllium and chromium or vanadium into one of the most precious of gemstones.
