This section is from the book "Turning And Mechanical Manipulation", by Charles Holtzapffel. Also available from Amazon: Turning and Mechanical Manipulation.

The split brilliant, or trap brilliant, figs. 1163 to 1165, only differs from the full brilliant, fig. 1161, in the foundation squares being divided horizontally into two triangular facets, forming an obtuse angle when viewed in elevation, as in fig. 1164. In cutting the split brilliant, the face of the stone is prepared by forming two bevels around the margin, which are afterwards converted into squares exactly like the stone trapped in two heights, fig. 1153. The upper height of squares is made narrower than the lower; the skill facets, and the brilliant facets, are cut in the same manner as for a full brilliant, and are both made to terminate on the ridge dividing the foundation squares into triangular facets.

The back of the stone is represented as trapped in three heights of different widths, the narrowest being nearest the culasse. The squares of the trapping are, as usual, placed opposite to the foundation squares on the front, and the row of squares adjoining the girdle has its angles replaced by eight under squares placed opposite to the brilliant facets on the front.

The double brilliant, or Lisbon cut, figs. 1166 to 1168, is a duplication of the brilliant cut, fig. 1161, having two rows of lozenge-shaped squares, and three rows of triangular facets. The two rows of foundation squares are first cut as for the stone trapped in two heights, fig. 1153; and they are afterwards converted into the lozenge shape, by cutting one row of skill facets around the table, one row of double skill facets upon the angles joining the two rows of squares, and one row of brilliant facets around the girdle. The dotted lines in fig. 1166 show the angles removed by cutting these facets, by which it will be seen that the row of double skill facets in the middle, removes at the one operation the inner angles of both rows of foundation squares.

Figs. 1163.

1166.

1164.

1167.

1165.

1168.

In some few instances a third row of foundation squares is added, by first cutting the stone in three heights, and then cutting three rows of double skill facets in addition to the row of skill facets around the table; but this degree of elaboration is considered to cause so much confusion in the rays of light, as to entirely neutralise the advantage obtained from the greater number of reflecting surfaces, and is seldom resorted to except for concealing defects in large stones.

The rose cut shown in figs. 1169 to 1173, as previously mentioned, is employed for those diamonds and other transparent stones that are too thin to admit of being cut on the back; and sometimes the rose cut is also employed for opaque stones that would derive no additional lustre from the back cutting. The true rose cut consists of twenty-four triangular facets, cut upon a stone having a circular or dodecagonal base, flat beneath and rounded at the top, as shown in fig. 1170. The central portion of the stone is cut into six equilateral triangular facets, the points of which meet in the center at the summit of the stone, and their bases form a regular hexagon, six other equilateral triangles are joined by their bases to the central facets, and their points rest upon the girdle. The spaces between the outer row of triangles are each cut into two triangular facets, which closely resemble the double skill facets in the brilliant cut.

Figs. 1169.

1171.

1173.

1170.

1172.

In cutting the stone, figs. 1169 and 1170, the back is flattened, and the face rounded to the general curve, the central zone of six facets is then cut, commencing with two opposite facets, and when these have been made parallel, and of the required size, the intervals are each cut into two facets, care being taken that all the facets are of equal size. The six principal facets in the outer row are next cut; and lastly, the six intervals between these facets are cut into six pairs of double skill facets, which complete the rose cut, as applied to diamonds, and thin gems generally.

Thicker stones, however, are sometimes cut with an additional row of twelve nearly square facets, arranged around the outer row of triangles, as in figs. 1171 and 1172; every square being equal in width to the base of its adjoining triangle; at other times, instead of squares, the outer row is composed of two rows of triangles.

The rose cut is also frequently applied to elliptical stones, as in fig. 1173; and, as there shown, the central hexagon is elongated, and the triangles are made of irregular forms.

Figs. 1174 to 1176 represent a stone cut in squares, or trapped in one height on both the front and back, which differ from each Figs. 1174.

1177.

1180.

1175.

1178.

1181.

1176.

1179.

1182.

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