Emerald fractures can be caused by the same things that give the gems their intense green color.
This is the chemical formula for beryl:
Beryl is composed of extremely rare beryllium (Be), aluminum (Al), silicon (Si), and oxygen (O). When the elements chromium (Cr) or vanadium (V) replace aluminum in the beryl formula, you get emeralds. The presence of these elements gives them their characteristic color. However, the valencies of chromium and vanadium don’t mesh well with the beryl lattice structure. Chromium and vanadium are heavier atoms than aluminum and strain the crystal structure.
Natural emeralds form underground in intense heat. As the replacement of aluminum takes place, gases escape. This creates microscopic voids in the crystals that can be filled with other minerals. These inclusions also strain the crystal structure and can cause emerald fractures. Sometimes, the gases themselves are trapped and form bubble or liquid inclusions inside the gemstones.
Emeralds aren’t the only members of the beryl family to undergo this process. Red beryl (bixbite) is formed when manganese replaces aluminum.
From the International Gem Society website