Beautiful question.

Emerald is a compound made of Beryllium, Aluminum, Silicon and Oxygen with the formula, Be₃A_l2(SiO₃)₆. The green color of Emerald is due to traces of Chromium present in them.

It is very interesting how different minerals and gems exhibit brilliant colors. Let's first understand why we see color around us. White light contains rays of all the colors. When white light is passed through a prism, one observes the separation of different colors contained in white light. A substance appears black because it absorbs all the different colors shone on it, whereas a blue substance absorbs everything except blue; what is left after absorbing is what one sees as color.

Many minerals in their pristine state are colorless, which means they do not absorb any color. They appear transparent. But, if some specific impurities are added to the mineral, the compound assumes an interesting color. The mechanism as to why a specific impurity produces a specific color is a question often asked by scientists, but the answer is complicated in most cases. But, for most well-known minerals, we know the reason why.

Emerald like many other gemstones such as ruby, sapphire owe their color to impurities present within them. In fact, Ruby and sapphire are made of exactly the same material (Corundum, which is a compound made of Aluminum and Oxygen with stoichiometry Al₂O₃), but ruby appears red and sapphire appears blue. The difference in color is due to traces of Chromium present in ruby, whereas blue Sapphire obtains its color from traces of Iron and Titanium impurities.

Emeralds are made from 4 elements that are found deep in the Earth’s crust: beryllium, aluminum, silicon, and oxygen. These elements are found flowing in veins filled with hot water called hydrothermal veins. When the conditions are just right in these veins and the cool down, the emerald crystals start to form. Emeralds can also be formed in magma rather than hydrothermal veins. A lot of the emeralds we mine today are the result of processes that happened hundreds of millions of years ago.