Can we talk abut precious gems without reference to ruby, emerald oralexandrite ? all of them have two things in common ; they are beautifl and hance valuable and each has a fraction of chromium (Cr) in them. Cr produces the purest red in ruby and red spinel, the most baeutiful green in emerald and a wonderful change of colour in alexandrite; green in day light and red in candle light or incandescent light.

Not only those, many other gem varieties get their beautiful green whaen a bit of cr is present in them, and most often refer to them with a prefix 'chrom'; chrom diopside, chrom tourmaline etc. Even without cr they are green but not the best green.

So would it not be 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.

In general ,electrons in atoms occupy certain orbital shells around the nucleus , like planets around the sun. Each shell represents an energy level. the innermost shell can contain 2 electrons, next shell upto 8, next upto 18 etc. The inner shells are full and generally outer shell contain a fewer electrons than it can accommodate. But this is diffferent in transition elements, inner shells may not contain full quota of electron (see figure 1-Cr). Cr can have upto 18 in third shell but it has only 12.

Electrons in most elements exist in these shells as pairs spinning in oppaosite directions. However in transition elements, in the incomplete shells many electrons will remain unpaired. When white light (dey light) falls on a gamstone, part of it is absorbed by the electrons and jump in to higher energy levels and the unabsorbad component of light reach us as the colour of the gemstone. Remember light is a from of energy and white light consists of different coloured waves as you see in the rainbow, each wave having a different colour and having a different colour and different amounts of energy. The amount of energy required by a paired electron is higher than that
required by an unpaired electron. Therefore it is the unpaired electrons of transition elements which absord part of light and cause colours in them. Most of gems get their colour due to transition elements but there are some which do not have transition elements but their colouring mechanism is different.

Now let us look at the role of the chromium gems. It can exist as a an impurity or as a part of the chemical formula. in ruby, emerald and alexandrite, Crexists as a substitute atom for aluminium atom.. Cr atom has six unpaired electrons, three involving bonding with other elements in the atomic structure and the other three changing energy levels by absording energy from white light.

Three of the unpaired electrons in Cr can occupy in different orbits or energy levels. The amount of energy required to raise an electron from ground level A to C is about 2.25 electron-Volts or eV which corresponds to yellow-green light and hence some electrons absorb yellow-green range of light(See figure2).

The amount of energy required to raise electrons from A to D leve is 3 eV and it Corresponde to violet part of visible light . So some electrons absorb violet colour. Millions of electrons in the gemstone absorb yellow-green and violet colours, so that stone appears in red.

Emerald ( Be3 Ai2 Si6 O18 +Cr)

The force thet surrounds cr (which has replaced Aluminium) is weaker than that of ruby. Therefore the energy levele of electrons are different. in this case about 2eV of energy, it is red part of light that is absorbed and the residual colour is the most beautiful pure green..

Alexandrite ( BeAi2 O4+Cr)
The forces that surrounds Cr is intermediate in Strength as compared with ruby and emerald. Therefore the C level is between 2 eV and 2.25 eV. The slight variations in the 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. The candle light or tungsten light is rich in yellow red component and hence stone appear in red in that light.

In general , electrons in atoms occupy certain orbital shells around the nucleus,like planets around the sun. Each shell represents an energy level. The innermost shell can contain 2 electrons, next shell upto 8, next upto 18 etc. The inner shells are full and generally outer shell contain a fewer electrons than it can accommodate. But this is different in transition elements in transition elements, Inner shells may not contain full quota of electron (see figure 1-cr). Cr can have upto 18 in third shell but it has only 12.

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