Most people are only familiar with the faceted form of emeralds, although they are also carved and cut into other shapes.
Uncut gems are usually referred to as gem rough, or in this case emerald rough. Gems also have a particular shape or “habit” they like to grow in. An emerald’s ideal habit is a six-sided crystal. An amazing specimen as seen from Wilenski shows the ideal shape and formation of gem-quality emeralds.
This particular example on the right is named “The Three Amigos” due to the similar quality and size of all three individual crystals. Most emeralds do not form this nicely and can look like green gravel when opaque and included.
Emeralds are complicated crystals, right down to their chemical formula Be3Al2(SiO3)6. They are a variety of the mineral species beryl, with pure beryl actually being colorless and referred to as Goshenite. The green color in emeralds comes from chromium or sometimes vanadium.
Emeralds are also considered a separate variety from green beryl. This gets very confusing for anyone unfamiliar with emeralds and green beryl since they are both green-colored beryl.
The official gemological distinction between the two is the “tone” of the color, specifically how light or dark the green is. Light green only qualifies as green beryl, and medium to dark green qualifies as emerald. For in-between colors the distinction can seem arbitrary, and the green cannot have too much blue in it. Otherwise the gem is an aquamarine, the blue variety of beryl.
There are a number of beryl varieties like Maxixe , Heliodor , Morganite , Bixbite , and Goshenite . The beryl species gets pretty complicated.
A rarity in the gem trade, trapiche emeralds are named after six-spoke grinding wheels used in South America. The one on the far right in this example image has an unusual double core.
Aside from the visible exception, a trapice forms six arms around a core. This is not to be confused with the phenomena asterism, a star effect shown in certain gems. Rubies and sapphires show six-rayed stars that move around on the gem, while the arms of a trapiche are stationary and not a phenomena.
Emeralds can show a phenomena called chatoyancy, or cat’s eye, which is an incredibly rare feature to find in trapiches.
The main commercial sources of emeralds are 90% from Colombia in South America, and 10% between Brazil in South America, and Zambia in Africa. Zambia is the larger producer of the two. There are other locations that produce them like North Carolina, USA, and Ethiopia, Africa. However, they are not the main suppliers of gem-quality emeralds on the international market.