Gem News
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.