Jeffries’ Square Brilliant

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

David Jeffries was the first writer to describe the Square Brilliant Cut. In 1750, when his Treatise on Diamonds first appeared, the Square Cut had been in fashion for about fifty years and was the dominant Brilliant Cut. Round, oval and drop-shaped Brilliants were also fashioned but were considered Fancy Cuts; to these Jeffries only devoted a single page.

Jeffries, a jeweler and a dealer in diamonds, was fortunate enough to live in London at a time when cutters in that city were famous for the quality of their work. He devoted himself to the study of diamond fashioning and discussed his theories in great detail with the master cutters, selecting for analysis only the most perfect Brilliants. The results were theoretical, in that they ignored the fact that cutters were obliged to produce the most profitable gem possible from each crystal or piece of rough. However, Jeffries was a pioneer in that he showed the way for both jewelers and laymen to discover ‘a well or even ill made Brilliant’. His fifty-five diagrams show ideally proportioned Brilliants for weights from 1-100ct. For each diamond he gave the correct depth and the correct culet size. A comparison of any Square Cut Brilliant would show whether it matched the weight indicated for its size or whether it was lumpy or spread.

Unlike that of the ‘Peruzzi’ design and the Round Brilliants, the table facet of Jeffries’ Square Brilliant is not a regular octagon. Instead, it has fourfold symmetry with the facet edges meeting alternately at 150° and 120°, a shape which goes suprisingly well with the outline. The facet edges forming the internal star are not straight, but bent at an angle towards the center of the gem and therefore not parallel to the other facet edges. The main angles of both crown and pavilion are 45°. The height of the crown and depth of the pavilion have a ratio of 1:2, resulting in a table size of about 56 percent. The girdle should be as thin as possible, though not ‘knife-edged’ (to avoid chipping). The size of the culet conforms with the results of the calculations made by Eppler roughly two hundred years later, i.e. 8 to 10 percent.