(via Diamond Cuts in Historic Jewelry:1381-1910) Herbert Tillander writes:
Mirror Cuts were more often oblong than square in outline. One of my experiments shows clearly why this was so. I produced replicas of the two halves of a large octahedron, as if the octahedron had been cleaved through the center. One of these halves I then fashioned into a square Mirror and the other into an oblong Mirror. In both, the aim was to produce maximum show with minimum weight loss. Both had full pavilions with relatively small culets. The results showed, first, that this type of rough, was ideal for Mirroring Spread Table Cuts and, secondly, how an elongated outline could be a good commercial propostion! They also showed why early cutters were tempted to produce gems with blunt corners.
Dimensions
Square: 2x2.8cm
Elongated Rectangle: 3.3 x 2.62 cm
Area
Square: 7.84 cm²
Elongated Rectangle: 8.65 cm²
Table size
Square: 88%
Elongated Rectangle: 81%
Crown angle
Square: 40°
Elongated Rectangle: 40°
Pavilion angle
Square: 49.5°
Elongated Rectangle: 51°
Culet size
Square: 15%
Elongated Rectangle: 17%
Weight
Square: 38.17ct
Elongated Rectangle: 43.52ct
I must admit that I did not succeed in obtaining ideal proportions, but then neither did the early cutters. I did discover that my oblong ‘diamond’ was 10 percent larger in area and 14 percent heavier than my square one, so one can understand why Mirror Cut diamonds were more often oblong than square.
It was, then, a lack of appreciation of fire—the separation of light into spectral colors—at that time, combined with the price factor, which encouraged the cutters to use rough from which it would have been impossible to fashion High Table Cuts without prohibitive loss of weight. Size was all important, reflecting, perhaps, the classical proportions discussed above. All this explains why, among old jewels, we find so many Mirror Cut diamonds with table sizes of up to 80 percent or more, but nevertheless with correctly proportioned pavilions.
An example of this is found in the pendant known as Palatine Lion (Pfälzer Löwe), one of the pièces de résistance of the Treasury in Munich. The diamond is just below the ring attached to the diamond-studded chain from which the pendant once hung. It has a distinct cleavage crack, found to be absolutely parallel with the octahedral face of a ‘was’. The fact that these are parallel confirms that this diamond can only have been fashioned from a triangular crystal of this type. Note also the obvious reason for the two blunted corners—a typical manner of achieving larger sizes at the expense of symmetry. Another example of a similarly inclined cleavage crack is found in a tiny (4x4mm) overspread Mirror Cut diamond, thte largest on the pedestal of a pendant representing Nessus and Deinarina.
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