DIFFERENTIAL HARDNESS OF DIAMOND DETERMINES THE FORM OF MOGUL CUT STONES: THE CASE OF THE KOH-I-NOOR

Every year some two million visitors flock to see the British Crown Jewels at the Tower of London and within lies arguably the world’s most famous diamond, the Koh-i-Noor or ‘Mountain of Light’ – its original Persian name seemingly at odds with the rather flat 105.602 carat oval stone that now sits at the centerpiece of the Queen Mother’s Coronation Crown. This stone however is the end product of an original historic ‘Mogul cut’ diamond which, when first displayed at the Great exhibition of 1851, all but failed to impress the expectant crowds with its lack of fire and brilliance.

Original depictions of the Koh-i-Noor are scarce and contradictory, and many questions remain on how and when this remarkable historical stone was cut and by whom. The ‘re-discovery’ in the Mineral collections at the Natural History Museum of a unique plaster cast commissioned by the British Museum in 1851 of the original diamond, and the subsequent creation of a later cubic zirconia replica was used in this crystallographic study to elucidate the nature of the style of the Mogul Cut. We know from those rare and brief historical descriptions that skilled Indian lapidaries were aware of ‘grain’. Stereographic projection identifying cleavage planes on the original form were used to re-orient the diamond within a cubic framework; following superposition of the differential hardness variations of diamond itself reveals that alongside the supposed original rough morphology, primitive tools and wear rates, directional hardness anisotropy was the major physical constraint leading to ‘Mogul cuts’ final form. We can infer that other Mogul and ‘Mogul style’ stones of this period such as the Taj-e-Mah and Orlov were cut in similar ways with similar constraints. Any Mogul style form that does not align to this cubic framework and resulting from directional hardness anisotropy could not be considered genuine.