In cutting the back, the stone is first rounded to the general contour; secondly, the culasse is cut; thirdly, the water basil for the row of squares adjoining the girdle; fourthly, the eight principal squares are cut on this row; fifthly, the eight squares adjoining the culasse; and lastly, the eight under squares, and the polishing completes the back.
The stone is again inverted upon the cement stick for cutting the facets on the front, and the table is first corrected, eight squares are then cut upon the water basil, exactly as for fig. 1151, as represented by the dotted lines around the upper half of fig. 1157; and the surfaces of the eight planes thus produced, constitute the surfaces of the eight principal planes in the finished stone, as shown in the figure. These planes are called foundation squares, and are converted into the pentagonal shape shown in the figure, by cutting one row of eight triangular facets around the table, and another row of eight triangular facets around the girdle. The bases of all the upper row of facets around the table, extend from the center of the upper edge of one foundation square, to the center of the upper edge of the adjoining square, and the apex touches the point of the pentagon upon the line of intersection of the two squares; thus removing the entire angle indicated by the dotted lines in the figure, and altering the position of the original octagon constituting the table of the stone, at the same time that its size is reduced by the removal of its angles. These triangular facets are called skill facets, from the difficulty of placing them correctly.
The lower row of eight facets around the girdle is produced by cutting a triangular facet upon every one of the lower angles of the foundation squares, every lower facet is placed exactly opposite the corresponding skill facet, and is extended perpendicularly until it reaches the point of the skill facet; but, in the half brilliant, the lower row of facets is not extended laterally to the middle of the foundation squares, as this would remove too great an angle, and diminish the size of the stone. The lower facets are therefore cut only so wide as can be ventured without interfering with the rounded form of the girdle. The dotted lines on the lower half of fig. 1157 are inserted to show that the lozenge is the primary form of the foundation squares in all the varieties of the brilliant cut, and the pentagonal form of these squares in the half brilliant arises simply from the lower facets being only partially developed.
In all cases of facetting transparent stones, it is very important that the foundation squares on the front should be equal in number, and exactly opposite to the principal squares on the back, and also that the lower facets should be opposite to the under squares, or otherwise the play of light would appear confused, and the brilliancy of the stone would be materially reduced. In order to place the squares opposite, it is quite necessary that both the front and back of the stone should be cut accurately with reference to the shape of the girdle, as the back of the stone requires to be entirely embedded in the cement at the time the front is cut; and the only guide employed by the lapidary for the correct position of the squares, is derived from the general shape of the girdle, and the certainty that the back squares have been correctly placed.
The full brilliant cut represented in figs. 1160 to 1162, may be received as the foundation from which the other varieties of the brilliant cut are derived, and is considered to be the most perfect form of facetting for gems. It is therefore almost invariably employed for all diamonds that are sufficiently thick, and perfect, to admit of its application; but diamonds, from their crystallisation in the form of the octahedron, generally have a nearly regular octagon for the table and base, instead of the irregular octagon selected for illustration. Diamonds cut into this form are called brilliants; and the term brilliant cut, when used alone, is always understood to imply that the front and back of the stone are both facetted, as shown in the figures. Other gems than the diamond, and also pastes, are however often facetted in the same manner on the front only, and the back is cut in some other mode that will permit of a greater number of facets being introduced, generally the trap cut, or the star cut, is employed on the back, and the stone is then said to have a brilliant cut front, and a trapped back, or star-cut back, as the case may be.
In cutting the full brilliant, the front of the stone is prepared by cutting the table, girdle, and water basil, exactly as for fig. 1151; and in forming the back, the eight principal squares are first cut just as in trapping a single height, the culasse is then cut, and lastly, the 16 facets around the girdle, called under facets, are produced by cutting two triangular facets upon every one of the angles of the eight principal squares, which thus become converted into the irregular pentagons shown in the figures.
In facetting the front, the eight foundation squares, and the eight skill facets around the table, are cut in just the same manner as for the half brilliant; but in removing the lower angles of the foundation squares, two triangular facets are cut at every angle, exactly the same as on the back of the stone. These facets are by some lapidaries called double skill facets, from being cut in pairs, and by others brilliant facets, because the rays of light are reflected from their surfaces with greater brilliancy than from any other part of the stone.
It will be observed that in the figures the lozenge shape of the foundation squares is irregular, the lower pair of sides being wider than the upper; this form is adopted partly to allow for the small portion enclosed in the setting of the stone, but principally in order to increase the play of light, by making the brilliant facets as large as can be ventured without materially impairing the symmetry of the cutting.