When one is looking at beryl gems, there is really no firm boundary between emeralds with a heavy blue tint or aquamarine with a heavy green tint and there is no firm boundary between the green to blue green of emerald and the green of green beryl. Strangely, this is the same oxide in the same percentage that imparts the red color to ruby when it is in the corundum lattice. In my gemology classes at the University of Nebraska, I utilize a small color set. These are available from GIA and are produced by Pantone. Emeralds are quite rare because of the rather limited geologic environment in which they can occur.
Many emeralds come from contact metamorphosed black shale beds.
Black shale is often the thinnest bed that is in a sedimentary rock sequence so there is little potential host rock for starters. Some emeralds form in limestone that has been subjected to contact metamorphism. Emeralds are usually clouded by many inclusions. The nature of the inclusions is usually indicative of the source of the emerald. Emeralds from Colombia normally have three phase inclusions.