The Rarity of Emeralds
Introduction
Emeralds are famous for their color and beauty, but how did Mother Nature cook up these little green wonders? It's a recipe calling for rare elements and earth-shaking events, and it changes from one exotic locale to the next. But there are key ingredients they all have in common.
The Ingredients of Emeralds
Emerald is a variety of the mineral beryl, which means that the first thing you need is a very rare metal called beryllium. Beryllium isn't something you just pick up at your local supermarket; it's found in less than three ten-thousandths of the earth's crust—0.0003%.
This means there are very few situations where you get enough beryllium together to even have a chance to make something more from it. But say you do get a dollop of beryllium in the right situation to whip up some beryl. Well, that's just half the battle. In order to make that beryl into an emerald, you also need a coloring agent, chiefly the elements chromium or vanadium. That's not so simple; these are also ultra-rare and most often aren't found anywhere near beryllium. Beryllium is low density, so it's typically located in the earth's crust, while chromium and vanadium are higher density and usually found much lower, like the mantle or the core. This is why you'll only find emeralds in some very special places around the world.
Colombian Emeralds
Let's start with one of the most special places: Colombia, where a unique process formed some of the finest specimens anyone has ever seen. Colombian emeralds are found along the eastern part of the Andes mountains, where several legendary mines lie in close proximity. Here, the emeralds are found in sedimentary rocks, which is very unusual. Typically, gemstones are found in igneous or metamorphic rock as a byproduct of extreme heat and pressure that thoroughly cooks the rock and recrystallizes it into something new. Instead, here it's less a roast and more like a ceviche. Hydrothermal fluids move through the cracks and crevices of black shale rocks, grabbing ingredients along the way and mixing them together to form something entirely new. This is a process known as metasomatism.
The Unique Formation Process
Remember how I said that beryllium and chromium are typically found in completely different parts of the earth? The black shale in Colombia is a major exception, and bits of both can actually be found in them. So the hydrothermal fluid just needed to mix them up and bam! Emerald! But not just your garden-variety emerald. See, the emeralds of Colombia are an extra special recipe in that they also contain virtually no iron. Iron makes up five percent of the earth's crust and is found in even greater amounts the deeper you go. Its presence also significantly alters the colors of gems.
The Role of Carbon
So how do these Colombian emeralds manage to ditch iron? Well, once again it all comes down to those special black shale rocks. In addition to the beryllium and chromium, they also contain a whole lot of carbon. As the hydrothermal fluid moves through the rocks, the carbon interacts with sulfide, a common compound in the fluid. This creates carbon dioxide, but it leaves behind sulfur. That sulfur combines with the iron in the ground to create the mineral pyrite. Pyrite is commonly found around Colombian emeralds or even as an inclusion within them. While the emerald itself does not have iron, it was all used up. Colombia may have won the geologic lottery with these black shale rocks; they're like a one-stop emerald specialty store with all the ingredients right at your fingertips.
Global Emerald Formation
The rest of the world wasn't quite so lucky, but still a great deal of fantastic emeralds are found around the world. Here, they're usually made with tried and true gemstone-making staples: extreme heat and pressure. Think magma and earthquakes—the same kind of geologic conditions that form mountains. These emeralds form in rocks that are categorized as mafic and ultramafic. These are igneous rocks that contain high amounts of iron and magnesium, plus the chromium needed to color emeralds. But what they don't have is beryllium. For that, we need another very different rock type to mix with these host rocks.
Mixing Rock Types
But how do you mix one rock with another? Well, it can happen when everything gets really hot and melty. The particular kind of rock we need to mix in here is a pegmatite, which is a granite with large crystals of quartz and feldspar. New chemically unique formations are made in the margins between the two rocks as hydrothermal fluids transport elements between them. In relatively small areas where the beryllium and chromium meet, emeralds can form. Remember when I said that those mafic ultramafic rocks were high in iron? Well, unlike Colombia, there are no special shales here to remove it. Instead, emeralds in the rest of the world contain varying amounts of iron, which affects their color in various ways. In Zambia, for instance, the emeralds are known for having a more blue-green tone, while Ethiopian emeralds have more of a grassy tint. Both of their colors are influenced by the iron in their crystal structure.
Other Emerald Sources
Zambia and Ethiopia are not the only countries whose emeralds were produced in this way. Countries like Brazil, Canada, Pakistan, Afghanistan, India, South Africa, and Russia also source emeralds from similar geological conditions.
Unique Emeralds from North Carolina
There is one last place that we're going to talk about that takes most of what we know about emerald growth and throws it out the window. It's right in our own backyard here in the U.S. of A: Hiddenite, North Carolina. North Carolina emeralds may not be plentiful, but they are well known for their quality.
The Special Conditions of Hiddenite
First, they're found in a special type of rock called a migmatite. Migmatite is a rock composed of both metamorphic and igneous rocks. This happens when a metamorphic rock is partially melted under high temperatures and the melted portion recrystallizes as an igneous rock. Many times this gives the rock a banded look where one color of banding represents the original metamorphic rock and the other color represents the recrystallized igneous rock, which is typically similar to a granite and contains a lot of quartz. What they don't contain is beryllium or chromium or vanadium. Instead, we know that these elements were brought into the migmatites via hydrothermal fluids, which form the emeralds in fractures and cavities in the rocks.
The Mystery of Hiddenite
But where did that fluid come from? Well, that's where the mystery begins. Remember when we talked about beryllium coming from granitic rocks deep in the earth? Well, geologists haven't found any of those anywhere near the Hiddenite emerald pockets, and the chromium or vanadium—the mafic ultramafic rocks that provided those elsewhere—aren't found in Hiddenite either. And there definitely isn't any of that magic Colombian black shale laying around. It's definitely a question for further study, and the answers may one day expand our understanding of what goes into making these little green wonders.
For more detailed insights on the rarity and formation of emeralds, you can check out the following resources:
No comments:
Post a Comment