It is one thing to identify an emerald versus similar looking green gems like peridot, garnets, or tourmaline. To figure out if an emerald came from Colombia or across the Atlantic from Zambia is a completely different story. This kind of detective work requires extensive training, a microscope, and prohibitively expensive lab equipment. Even with all these tools, lab gemologists are limited in conclusively identifying emerald sources.
There are two main methods used to determine the origin of an emerald, the same as with most other gems. One is Raman spectroscopy, which any professional lab uses. While the technical application and data processing is very complex, the short version is scientists send a laser through gem material (it has to be see-through for the laser), and the computer reads the light that comes out of the gem in terms of a rainbow. The computer then processes what elements are in the “unknown” gem material, and gives details about what concentrations the different chemicals are present in the gem. This is how the lab gemologists determine chemical composition without damaging these precious stones.
If you are confident in reading the spectrum directly, using a cheap pocket spectrometer to identify the gem is an option too. Just note that you cannot process all the details the way a machine can, and make sure you get a strong light source through the gem. The spectrum does not always show up perfectly either, and can take training to read it this way.
Chemical composition alone is not always enough to determine origin. For that, the lab compares the information collected from that particular emerald to Colombian emeralds, Brazilian emeralds, Zambian emeralds, etc. In essence, they check their database for emeralds with similar compositions. After comparing all these emeralds the person reading the data gets an idea of where the emerald is most likely from, but this is not always a guarantee of emerald origin.
Having this database is no small feat either. Each gem sample needs to be confirmed for not only their country of origin, but which specific mines they come from (when possible). This means assembling precise data for dozens, if not hundreds of other samples from all different sources globally. The more information in the database, the more accurate the result will be… and the more expensive it is to put that database together.
The one definitive distinction the spectroscopy gives about emeralds is between naturally forming ones and synthetically made ones. There are always trace amounts of various elements in natural emeralds, since nature does not think twice about mixing whatever is in the ground together. What comes out of areas estimated to have gems is nothing less than a gamble. These trace elements are absent in synthetic emeralds, and many usually have chromium and vanadium levels that far exceed their natural counterparts.
There are also a number of diagnostic inclusions in synthetics, though not all synthetics show their features clearly (hence all the gem labs).
Emeralds from different locations can have very similar chemistry. Because of this, scientists also check the inclusions, the other stuff mixed into emeralds.
Some inclusions are region-specific and diagnostic, meaning the stuff can’t be found in emeralds from any other source. These are not always present in the emeralds, possibly making the origin inconclusive if there is not enough other evidence. However, the lab should have a good idea where the emerald comes from at this point.
If nothing else, if you are looking at a vividly green gem that is highly included with features like three phase inclusions, fractures, needles, crystals, etc, then you are most likely looking at an emerald. Plus, the shimmery green color is pretty distinctive against other gems, though this does not show on camera well.