by Allan Taylor

Created on: May 29, 2010   Last Updated: September 16, 2011

Why are natural emeralds so rare in occurrence compared to beryl and aquamarine, which are the same mineral but a different color?

Why are beryls and aquamarines found  in  pegmatite deposits but rarely so are emeralds?  If you are a prospector of gemstones and of emeralds you should know the reason before venturing forth.

To understand the occurrence of emerald and beryl in general you have to  study the chemical composition of the mineral

beryl and what causes the color.   Beryl is a beryllium aluminum silicate which if it had no coloring impurities would naturally be colorless, and many are, such as the  colorless gem beryl called “goshenite”.   These crystals may have lots of alkali ions and water molecule impurities but these don’t cause any color.

The color of beryl crystals is due to the presence of transition metal ions substituting for Al in the octahedral sites of the crystal structure.   The most common metal substitution is both ferrous and ferric iron, since iron is fairly ubiquitous element in the earth’s crust.   Both types of iron can substitute for Al,  and ferrous iron can also occupy channel sites, also Al can substitute for silicon.  Any charge unbalance can be made up by addition of alkali ions (Na, K, Rb, Cs) which being large,  fill up in channel positions.  This causes an increase in density (specific gravity).   The specific gravity of beryl is quite variable (S.G.  2.65  to 2.91)  unlike quartz (S.G. 2.65) which is constant, due to it having hardly any ionic impurities.  The various combinations of ferrous and ferric ion cause the blue and different shades of green and yellow colors of beryl (1).   

The valuable blue-green and grassy green variety of beryl is what we call emerald (ancient definition).    The color is usually due to a small amount of chromium (ca 0.2 to 2%   Cr2O3)  in  its composition.   However,  In nature you can find emeralds colored by  vanadium alone or with both vanadium and chromium so the definition of what is an emerald has been controversial.   Personally,  I prefer the simple definition given above,   since we call yellow beryl “heliodor” and pink beryl “morganite” without worrying about what impurities are present, even if we knew.  More on this later.

• 1
• 2
• 3
• Next Page >>