Beautiful question. 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.

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.

An emerald is a type of mineral and there are many different types of minerals. Nearly all the rocks that you see on the surface of the Earth and also the rocks very deep in the Earth are made of different minerals. Some minerals are very common and other minerals are very rare. To understand this, we need to understand what minerals are.

Minerals are made from different types of atoms. You can imagine they are made of different types of Lego pieces. Now with Lego, two things can happen. Firstly, imagine you have three blue and three white Lego pieces. You can build many different forms with them. Similarly, with the same type of atoms, you can build many different minerals. Secondly, imagine building a house with Lego pieces. You can build the same house out of blue and yellow pieces, or green and white pieces or black and red pieces. Similarly, you can make the same shape with different types of atoms. This will also make different minerals. Does that make sense? Then you can understand that for a certain mineral to form, we need to have the right type of atom and the right structure.

Emeralds need a type of atom which is called Beryllium and Beryllium is very rare. Only in few places can you get enough Beryllium together to make emeralds. Also, in order to get the correct structure, you need to have the correct temperature and pressure. The temperature and pressure increases deep in the Earth. The deeper you go, the hotter it gets and the higher the pressure. Emeralds form deep in the Earth in locations that have just the right temperature and pressure and that happen to have enough beryllium.

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.

Emerald is a type of crystal of a mineral called beryl which forms in large-crystaled granites and some kinds of metamorphic rock, produced by the heat and pressure deep within the earth. Emerald is different from other kinds of beryl (such as aquamarine) by containing small amounts of the element chromium.