Due to the conditions under which they form, most emeralds contain numerous fractures and openings. This photo shows the classic three-phase inclusion of a Colombian emerald, consisting of a negative crystal filled with a gas bubble, a halite crystal and a brine solution. If cutting exposes this inclusion to the surface, it will fill with air, producing reflection. Such inclusions are typically filled with oil or resin to reduce the amount of reflection.


More information on emerald inclusions is available at http://www.cigem.ca/inclusion/em01.html