Emerald is the green variety of beryl, a beryllium aluminium silicate, coloured by traces of chromium and sometimes vanadium. The same mineral species gives aquamarine when coloured by iron, morganite when pink, heliodor when yellow and goshenite when colourless; they are one mineral wearing different impurities.
Emerald's defining practical characteristic is that it is almost always included. The internal fissures and wisps are so universal that the trade calls them the jardin, the garden, and treats an eye-clean emerald as suspect rather than desirable. The reason is geological: emerald requires beryllium and chromium, elements that occur in quite different rock types and rarely meet, so the crystals grow in disturbed, chemically hostile settings full of fractures. Beryl has a hardness of around 7.5 to 8, but emerald's fractures make it far more fragile than that figure suggests, and it is not a robust stone for daily wear.
Nearly all emerald on the market is oiled, fractures filled with cedarwood oil or resin to reduce their visibility. This is a long-standing and generally disclosed trade practice rather than a fraud, but it is a treatment, it degrades over time, and it is why emeralds should not be cleaned ultrasonically.
The historical sources were the Egyptian mines in the Eastern Desert near Mount Smaragdus, worked from the Ptolemaic period and traditionally associated with Cleopatra, producing generally poor material. The Spanish encounter with Colombian emeralds in the sixteenth century transformed the gem entirely; Muzo and Chivor still produce the finest stones, and Colombian emeralds flooded into Mughal India and Ottoman Turkey through Spanish trade, which is why great historical emeralds in Asian collections are usually American in origin.