Most natural emeralds are made in geothermic phenomenons called hydrothermal veins. Hydrothermal veins are cracks in the rocks deep in the earth. As magma flows deep underneath the ground, super heated liquids migrate out from the magma and flow through the cracks in the rocks. These cracks become hydrothermal veins. With these super heated fluids also come mineral deposits that travel with the fluid until it has cooled enough to turn into a crystallized form. These deposits react with the minerals around them to form natural emeralds, as well as many other types of gems depending on the minerals involved.
Emeralds are usually formed when the element beryllium combines with chromium. The chromium is what gives emeralds their green color. If the emerald is formed by porphyroblastic growth, there may also be hints of blue, as well as the color may vary from light green to a very dark green. This is due to influencing minerals from the host rock.
Porphyroblastic growth is another way that emeralds may form. In this case, the emerald forms inside of a host rock. As the rock takes shape from the cooling magma, the emerald inside removes all the minerals it needs from the surrounding rock and forms the emerald inside. These emeralds are not as clean and clear as those formed in hydrothermal veins as they usually include some of the crystal elements of the surrounding rock.
Emeralds can also be grown in a lab. Labs do this by creating the same conditions in the lab that beryllium and chromium would undergo in a hydrothermal vein. A flux solution may also be included in the mix to help the two elements combine. Because the conditions are controlled, the clarity, color and strength of the emeralds grown in labs are usually much better than those found in the wild. However, a trained gemologist will still be able to determine whether or not a gem has been lab grown or was found wild in the quarries of Columbia, Africa or Brazil.