April 12, 2015

Good afternoon everyone! Have you ever wondered why emeralds gemstones are so expensive? If so, I would like to tell you the reason.  


But first I need to talk to you about a great family: the BERYLS. You probably don't know them as beryls, but if you have ever heard about emeralds, aquamarines, heliodors, etc.…. then you are aware of BERYLS.


For those of you who enjoy chemistry they are known as beryllium aluminium cyclosilicate, -  Be3 Al2 (Si6 O18).


 And when beryls are completely pure - that is there are no foreign elements or agents – they are colourless and are called Goshenites. 


The question that generally comes next is: Why are some beryls called Emeralds, and others called Aquamarines?


This is due to some elements known as cromophores, which are foreign elements, diminutive intruders that sneak into the purity of this formula and totally change what we see. If the intruder is chromium or vanadium, we get an Emerald, if it is iron then it can be either an Aquamarine, Green Beryl or Heliodor (depending on the valence); if it is manganese, we are left with a Morganite or Red Beryl.


Does an Emerald cost the same as a Green Beryl? Both of them are green. The answer is no, not at all. Why? I know, I am continuously saying the same thing: Beauty, durability and RARETY, the three characteristics of a gemstone; notice how rarity is in capitals.


Now I will explain why the emerald is one of the most expensive gemstones in the world:


Chromium (the emerald’s cromophore) is concentrated in the mantle and it is associated with basic rocks (those poor in silica content). Unlike beryllium that is concentrated in the crust and is associated with acid rocks (rich in silica content), so these two elements coinciding are not very frequent.




But things don't end here. The intruder, that is, the atom of chromium, then replaces an aluminium atom, and the more chromium the greener and more intense the colour will be, but the loss of alumina makes the structure weaker, making the emerald more fragile than other beryls.


If to this we add the force that these stones withstand during the formation process- these gemstones originate from metamorphic rocks, that is, rocks that suffer big changes of pressure and temperature- we will understand the reason for all the fissures that are usually found in emeralds and that don't appear in other beryls.


And how can we identify the cromophore in gemstones, and separate green beryl from an emerald? By using the spectroscope; it is one of my favourite tools.




With thanks to Mr Colin H. Winter (a great Spectroscopy Master).


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