สำนักราชบัณฑิตยสภา

The Journal of the Royal Institute of Thailand Vol. 27 No. 3 Jul.-Sept. 2002 A Ruby Grading System ˜ÙÙ 2.3 Cut grading Cutting is the third most important factor when evaluating rubies. The cut component is worth 10 to 20 per cent of the value in a grading system. Criteria for cut grading are the proportion and finish of a ruby. Proportion: There are three criteria used to assess propor- tion. 1 -Face-up balance When viewed in the face-up position, the opposite parts of the faceted stone should be exactly the same size and well shaped. The overall effect should be appealing to the eye. 2 -Profile balance When viewed from the side of the table, the culet should be exactly cen- tered, the girdle not wavy, and the pavilion bulge should be even. Total depth percentage (the height of a ruby measured from table to culet, divided by the width and multiplied by 100) should normally be between 60 and 65 per cent. The crown height, measured from the table to the girdle, should be between one-fourth and one- third of the total depth. The pavil- ion depth, measured from the girdle to the culet, should vary be- tween two-thirds and three-fourths of the total depth. (Figs. 12-13) 3 -Brilliance The light re- turned to the eye is known as bril- liance. As the brilliance increases rubies become more valuable. When light leakage occurs it is known as windowing. When dark areas are visible, it is know as ex- tinction. Finish : There are three criteria used to assess finish: 1-Polish: regards the sur- face characteristics 2-Symmetry: considers the shape, position, and arrangement of facets 3-Facet survey: considers the number of facets. The posi- tion of the facets should be suit- able to the cutting style. (Table 3) 2.4 Weight (in carats) will also determine the price of the stone. A large-sized ruby should have a higher price per carat than Fig. 12 Fig. 13 Table 3 Cut grading Four criteria : 1. Face-up balance ............ 10 points 2. Profile balance ................ 10 points Grade by nake eye 3. Brilliance .......................... 5 points 4. Finish ............................... 5 points Grade by 10x loupe (face-up balance + Profile balance + Finish Cutting grade = + Brilliance 5 Max. score = 10 Min. score = 1 ➤ ➤ Cutting grade Grade Point Excellent 10-8 Fine 8-6 Very good 6-4 Good 4-2 Fair 2-1