The emerald’s dark green color and shine have long made it a coveted and valuable stone. The first known emerald mines were in Egypt, dating to the fourth century BC, and Cleopatra used emeralds in her jewelry. The ancient Greeks likewise prized it, and gazing upon it was thought to relieve eye problems.
Emeralds are a type of beryl, a mineral group that also includes aquamarine and other semi-precious stones. All beryls are created from beryllium, aluminum, silicon, and oxygen. They would be nearly colorless unless other elements were included. In emeralds, small amounts of chromium or vanadium create the green color.
These elements can’t combine in a random manner to form an emerald. Rather, they need to combine in a rigid structure called a “matrix” or “crystal lattice”.
The best way to understand crystal formation is to think of each molecule or atom as a brick in a dump truck. If the truck dumps its load, you’ll have a pile of bricks with no structure. Imagine that, under certain conditions, the bricks would connect to each other in specific ways, long side to long side, short side to short side. If you dumped them in a thick liquid like oil, they’d line themselves up as they descended, forming an ordered network that clings tightly together.
Some substances, like salt, form matrixes easily. If you create a salt solution so strong that no more can be dissolved (a “supersaturated” solution), a single salt grain can be the “seed” that allows other molecules to form a very large crystal around it as the water evaporates. The compounds that form an emerald, however, won’t form a crystal unless some very specific and extreme conditions are met.
WATER AND HEAT
Emerald formation is “hydrothermal”, meaning it requires water and heat. Water seeping down through the earth dissolves minerals, and when it trickles down deeply enough it reaches “magma”, which is superheated liquid rock. Water, carbon dioxide and volatiles are then expelled as the magma cools and the water becomes superheated.
The fluids are forced upward by their expansion, and squeeze through fractures in the surrounding rock. Since these liquids are rich in minerals, when they begin cooling some of the minerals form crystals within the cracks. If the conditions and mineral balance are just right, emeralds can form.
Since the cracks these fluids follow can occur in almost any rock, emeralds can be found in and around beds of igneous (like granite, which is formed by rapidly cooling magma), sedimentary (like limestone, formed by eons of matter that settles and compresses), and metamorphic stone (formed when sedimentary stone is compressed to the point it heats and begins melting).
Sometimes magma rising into the crust will contact the water, forming a super-rich mineral concentration that then forms dense veins of stone and crystal called “pegmatite”. Emeralds, aquamarine, tourmaline, topaz, fluorite, and apatite are all found in pegmatites.
Beryls are quite common, and the New England area of the U.S. has produced the largest beryl ever (about the size of a box-car!) but few emeralds. Colombia, however, is the most productive emerald producer in the world, with gems found in veins where ancient hot water flows penetrated masses of limestone.
Emeralds can be synthesized today by mimicking the ancient processes, but these processes all start with a simple beryl seed crystal with the “emerald” matrix deposited on top. The result is not a pure emerald, but rather a “composite” that looks like an emerald to everyone but a jeweler.
Even with this understanding of emerald formation, they’re still a mystery. How the conditions, chemical balance, and correct cooling conditions can occur is not thoroughly understood.