John Mawe’s Blunders

(via Diamond Cuts in Historic Jewelry:1381-1910) Herbert Tillander writes:

For many years I wondered who had first introduced the idea that, for a Brilliant to be correctly shaped, it was desirable that it should have an overall height equal to its wideth. Then I came across the following passage from the second edition of John Mawe’s Treatise on Diamonds: ‘The rule to be adopted in regulating the height of the brilliant is (supposing the stone to be a regular octahedron), to divide it into eighteen parts. Five-eighteenths are cut away to form the table, and one-eighteenth for the collet, which will reduce the height one-third, and the diameter of the collet will be one-fifth of the table. If these distances are preserved, the collet will play in the center of every facet, but if there is any variation, it will play higher or lower, and greatly diminish the intensity of luster...’

Mawe cannot have meant an octahedron, but rather a bipyramid reduced in advance to a shape with an overall height equal to its width—he goes on to repeat the rules of Jeffries and to give additional proof, both in his illustration of a correctly proportioned Brilliant and in his own text: ‘the inclination of the facets to the girdle ought to be 45°, and the bizel should be inclined to the table at the supplement of the same angle.’ His first statement is an obvious misprint or oversight, yet no writer appears to have noticed it, not even Paul Grodzinski who, in his reprinted edition of Mawe’s Treatise, comments on a number of other details but not on this. Many other writers have simply accepted the error, believing that Mawe was referring to early Brilliant Cuts with octehedral main angles. But it is obvious that a stone of this sort could have been fashioned only in the very rare instances where the rough stone was a regular octahedron. No cutter would ever have started fashioning a stone by transforming an irregular crystal into an octahedral shape, thereby considerably reducing its weight. The old rules remained in force and no changes in standard proportions were made until the late nineteenth century.

Of course, cutters did not always observe the rules for ideal proportions, but even the earliest Square Cut Brilliants were hardly likely to be fashioned from octahedral rough; for the most part they were refashionings of obselete cuts. If an old square-shaped Point or Table Cut displayed satisfying light effects, the gem was simply faceted into a Brilliant without changing the proportions. It is possible, presumably, that such recuts may occasionally have served as prototypes for fashioning directly from octehedral rough.