The occurrence of emerald is relatively rare because this mineral is only formed under very specific and unusual geological conditions.
Like aquamarine (sea blue-green), heliodor (golden), morganite (pink) and red beryl, emerald is a member of the beryl family. Beryl, Be3AL2Si6O18 is an allochromatic mineral this means that it is colourless in its purest form (Goshenite). The wide range of colourful gems that make up the beryl family owe their attractive hues to trace elements or chromophores. In the case of emerald this involves a trace of chromium (or vanadium) that replaces a small amount of the aluminium present in the beryl formula. However this is not really so simple as it may sound since the coexistence of beryllium and chromium is rarely encountered in nature.
The fact is that chromium precipitates early on in igneous activity and the element is therefore most abundant in mafic and ultramafic rocks.
Beryl on the other hand is a late-stage mineral that is generally to be found in or closely associated with pegmatites and hydrothermal deposits. The majority of emerald deposits are therefore to be found in the contact metamorphic schists created when pegmatite dykes penetrate earlier chrome rich host rocks. Schistose emeralds generally occur where beryllium has escaped the intrusive body and been transported into the Cr bearing contact metamorphic zone in the form of liquids or vapours.
Chrome is generally accepted as being the primary chromophore for emerald though several major American laboratories have stated that they consider vanadium to be an acceptable alternative.