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Chromium - The King Maker


Minute amount of  Chromium in  Ruby, Emerald or Alexandrite -  produces the purest  red in Ruby and red Spinel, the most beautiful green in emerald and a wonderful change of colour in alexandrite -  green in daylight and red in candlelight or incandescent light.

 

Many other gem varieties also get their beautiful green when a bit of cr is present in them, and most often refer to them with a prefix  'chrome' -  Chrome  diopside,  Chrome  tourmaline  etc. Even without cr they are green but not the best green.

So its interesting  to know a little about chromium and appreciate its role as "king maker". Most Gemstones get their colour due to the presence of transition elements in them: v, Cr, Mn. Fe, Mn, Fe Co, Ni, Cu.etc.

like planets around the sun. When white light (daylight) falls on a gemstone, part of it  jumps into higher energy levels and the unabsorbed component of light reach us as the colour of the gemstone. Most  gems get their colour from transition elements but there are some which do not have transition elements and their colouring mechanism is different.

 

Chromium gems  can exist as an impurity or as a part of the chemical formula. in Ruby, Emerald and Alexandrite Cr exists as a substitute atom for aluminium atom. So some electrons absorb violet colour. Millions of electrons in the gemstone absorb yellow-green and violet colours, so that the gem appears Red.

 

Emerald ( Be3 Ai2 Si6 O18 +Cr)

 The Red part of light  is absorbed and the residual colour is the most beautiful pure green.

 

Alexandrite ( BeAi2 O4+Cr)

 Slight  variations in  absorption can cause different colours. In day light high energy blue-violet components of light is higher and therefore the stone is green in daylight. In  candle light or tungsten light its rich  yellow- red component makes it  appear  Red. 

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POLYASTERISM - Double Stars /12 Rays


Polyasterism is the display of more than one star within a narrow angle of observation; they aren't stars of same star-network.  Rarely Sri Lankan star sapphire produce  display multiple stars or polyasterism. Relatively common two-stars are often called "Siamese twins" and rare three-stars "triplets" in the trade.

They not only comprise 6-ray stars but also 12-ray stars, prevalent among blue star, star ruby, and all other sapphire colors, sometimes fetching higher prices than that of single star stones.

 

Morphological disturbances in the single crystal or forming a compound crystal by bonding/fusing/cementing several crystals together  by geological reasons or Branching/penetrating of crystals are the two main causes that convert a star sapphire into a polyasteric stone.

 

Displacement of two or more sections by glide causes display of asterism independently in the individual sections arising from their rutile needle inclusions - this is the polyasterism in single crystals. Similarly compound crystal formed by aggregation of crystals display the effect.