Small unworked surfaces of the rough diamond are termed natural crystal faces (naturals); they occur in cut diamonds mostly beneath the girdle, occasionally also on the point. These natural crystal faces are left by the cutter in order to save weight.
The brilliant cut has in general been developed from the crystal form of the octahedron. The girdle plane then lies parallel to the plane of the square cube face of the octahedron. In rounding the brilliant the cutter tries to keep the circumference of the girdle as large as possible and therefore keeps to the very outermost edges of the rough stone (Fig 309). Tiny parts of the octahedral faces may then remain unworked, so that up to four natural crystal faces may be left on a brilliant.
The surface of "naturals" exhibit very typical structural features in the form of fine parallel lines or three-cornered figures (so-called "trigons", rarely tiny rectangles or squares, which prove it to be a natural crystal face of the diamond (Fig 310-311).
|Fig 309 Naturals occur on the cube faces of the octahedron|
|Fig 310 Typical crystal face of a "natural" with trigons||Fig 311 Naturals at the girdle with trigons and line|
These natural crystal faces are usually very small and lie immediately below the girdle. So long as they are not visible when looking at the crown they are ignored in cut-grading. Only when they impinge on the circular shape of the girdle 'or extend over more than a quarter of a pavilion facet are they regarded 'as down-grading the cut (Fig 312-318).
|Fig 313 Natural Face||Fig 314 Natural Face|
|Fig 315 Natural Face||Fig 316 Natural Face|
|Fig 317 Natural Face||Fig 318 Natural Face|