Beryl samples from Badmal mines, Orissa were studied by electron spin reson
ance, optical absorption and Mossbauer spectroscopy. It was found that on a
rtificial irradiation, colourless beryls from Orissa turned to shades of ye
llow-green. On subsequent heating at 300 degrees C, the hue changed to yell
ow and upon further heating at controlled conditions they turned to a more
desirable sky blue, accounting for value addition. Continued heating above
500 degrees C, however, rendered them colourless. Similar studies were also
carried out on natural yellow, blue and green beryls from Orissa, all of w
hich acquired a greenish hue on irradiation and turned colourless on heatin
g to >5000 degrees C. Electron probe microanalysis of beryls showed iron as
the major transition element impurity. ranging in concentration between 0.
5 to 0.7wt%. Investigations using electron spin resonance, optical absorpti
on spectroscopy and Mossbauer spectroscopy reveal that: colours produced we
re on account of Maxixe-type defect centres as well as radiation-induced ox
idation of Fe2+ to Fe3+ ion and associated charge transfer process. It is i
nteresting to note that the yellow and blue colour produced on irradiation
as well as post irradiation heating showed similar hue to that of the natur
al ones, both having formed on account of similar charge transfer process.
Thus, it can be inferred that natural yellow, blue and green colours in ber
yls comprising low iron impurities are due to prolonged irradiation in natu
re.