of his
co-workers, it has been found that luminescence is not due to
extraneous impurities, but is a property of diamond itself. This
indication is further strengthened by the definite correlation which is
found between luminescence and other properties, which differentiate
the allotriomorphic modifications of diamond. Fractured edges of
cleavage plates of diamond luminesce far more strongly than the
polished faces of interior of the crystal. So the inference is that
luminescence is excited by purely physical cause, namely a disturbance
of the regularity of the crystal structure. The variations in intensity
of the luminescence may be explained as due to variation in the number
and extent of such irregularities within the diamond. It is also
evident that an intimate interpenetration of positive and negative
tetrahedra in diamonds of lower symmetry would constitute a disturbance
in the regularity of crystal structure.
*
'It has been found unnecessary to postulate the presence of extraneous
impurities in the diamond, and if such impurities were the cause of
luminescence, it would be difficult to understand why diamonds having
the higher type of symmetry do not exhibit luminescence to anything
like the same extent. On our present views, the latter fact finds a
natural explanation in the circumstance that such uiterpenetration does
not exist in the octahedral variety of diamond., The chemical or
impurity theory is also discredited by the observation that strongly
blue luminescent diamonds are often of highest quality in respect of
transparency and freedom from colour.'
Size of diamonds.—The
average size of diamonds obtained in different countries varies
considerably. When India and Brazil were the only countries producing
diamond, stones of more than 20 carats in weight were a rarity, and
* Raman, Sir C. V., and others. The structure and properties of diamonds, Current Science, Vol. XII, No. I, pp. 33-42 (Supplement), 1943.
71
