Summary
Question: Aquamarine and most other members of the beryl family have nice big, clean crystals. Why are emeralds different in general? What causes emerald fractures? No one has given me a good answer yet. Isn’t a beryl a beryl?
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faceted emerald

Emerald, 7.65 cts. Photo by Dennis Harper. Licensed under CC By-ND 2.0.

Answer: The same things that give emeralds their intense green color can also cause their fractures.

The Chemical Basis for Emerald Fractures

This is the chemical formula for beryl:

Be3Al2Si6O18

Beryl is composed of aluminum (Al), silicon (Si), oxygen (O), and extremely rare beryllium (Be). When the elements chromium (Cr) or vanadium (V) replace aluminum in the beryl formula, you get emeralds.

The presence of these trace elements gives emeralds 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.

How Emeralds Form Underground

Natural emeralds form underground in intense heat. As the replacement of aluminum takes place, gases escape, creating microscopic voids in the crystals. Other minerals can fill these voids, creating inclusions that also strain the crystal structure and can cause emerald fractures. Sometimes, the gases themselves remain 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.

Roy Slack

Emerald crystal - Colombia

Emerald crystal, Muzo Mine, Mun. de Muzo, Vasquez-Yacopí Mining District, Boyacá Department, Colombia. Photo by Géry Parent. Licensed under CC By-ND 2.0.