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The Journal of emmoemmoemmo Volume 28 No. 4 October 2002 Development of concave faceting Verneuil > * synthetic sapphire Gem-quality spessartine- grossular garnet The Gemmological Association and Gem Testing Laboratory of Great Britain Gemmological Association and Gem Testing Laboratory of Great Britain 27 Greville Street, London EC1N 8TN Tel: 020 7404 3334 Fax: 020 7404 8843 e-mail: [email protected] Website: www.gem-a.info President: Professor A.T. Collins Vice-Presidents: N. W. Deeks, A.E. Farn, R.A. Howie, D.G. Kent, R.K. Mitchell Honorary Fellows: Chen Zhonghui, R.A. Howie, K. Nassau Honorary Life Members: H. Bank, D.J. Callaghan, E.A. Jobbins, H. Tillander Council of Management: T.J. Davidson, R.R. Harding, I. Mercer, J. Monnickendam, M.J. O'Donoghue, E. Stern, I. Thomson, V.P. Watson Members' Council: A.J. Allnutt, S. Burgoyne, P. Dwyer-Hickey, S.A. Everitt, J. Greatwood, B. Jackson, L. Music, J.B. Nelson, P.G. Read, P.J. Wates, C.H. Winter Branch Chairmen: Midlands - G.M. Green, North West - D. M. Brady, Scottish - B. Jackson, South West - R.M. Slater Examiners: A.J. Allnutt, M.Sc, Ph.D., FGA, L. Bartlett, B.Sc, M.Phil., FGA, DGA, S. Coelho, B.Sc, FGA, DGA, Prof. A.T. Collins, B.Sc, Ph.D, A.G. Good, FGA, DGA, J. Greatwood, FGA, G.M. Howe, FGA, DGA, S. Hue Williams MA, FGA, DGA, B. Jackson, FGA, DGA, G.H. Jones, B.Sc, Ph.D., FGA, Li Li Ping, FGA, DGA, M. Newton, B.Sc, D.Phil., C.J.E. Oldershaw, B.Sc. (Hons), FGA, DGA, H.L. Plumb, B.Sc, FGA, DGA, R.D. Ross, B.Sc, FGA, DGA, P.A. Sadler, B.Sc, FGA, DGA, E. Stern, FGA, DGA, S.M. Stocklmayer, B.Sc. (Hons), FGA, Prof. I. Sunagawa, D.Sc, M. Tilley, GG, FGA, CM. Woodward, B.Sc, FGA, DGA The Journal of Gemmology Editor: Dr R.R. Harding Assistant Editors: M.J. O'Donoghue, P.G. Read Associate Editors: Dr C.E.S. Arps (Leiden), G. Bosshart (Zurich), Prof. A.T. Collins (London), Dr J.W. Harris (Glasgow), Prof. R.A. Howie (Derbyshire), Dr J.M. Ogden (Hildesheim), Prof. A.H. Rankin (Kingston upon Thames), Dr J.E. Shigley (Carlsbad), Prof. D.C. Smith (Paris), E. Stern (London), Prof. I. Sunagawa (Tokyo), Dr M. Superchi (Milan), CM. Woodward (London) Production Editor: M.A. Burland Vol 28, No. 4, October 2002 ISSN: 1355-4565 Development of concave faceting of gemstones A.D. Morgan Birmingham, West Midlands ABSTRACT: A brief review of gem faceting methods intended to enhance the visual appearance of the stone is introduced, with illustrations of stones with various special cutting designs. Machines recently developed for producing concave facets on gem stones are illustrated and discussed. A detailed description of a machine for amateur use developed by the author is given, and the problems encountered in this method of faceting are outlined. The result of an experiment with a specially cut triangular prism having simple flat and cylindrically concave facets to compare the refracted ray patterns is shown, together with the computer-calculated theoretical ray path of a pencil of light impinging on a combination of concave lenslets and convex mirrors. Two concave faceted stones cut by the author, one in CZ and one in fluorite are featured and the patterns obtained with a Nelson Gem Fingerprinter are illustrated. 193 193 Keywords: Concave facet, faceting, gem cutting machines placing and polishing of flat facets. This has "is it original resulted in the development of standard cuts for round brilliants and other shapes to it was once I answered truthfully give maximum brilliance and weight yield and may be again" from the rough crystal. Further developments include the introduction don marquis, 1930 of the Profile (Princess) cut by Arpad Nagy (1960), which has a series of V-grooves cut Introduction into the back of the stone, enabling flat crystals to be cut economically, and the he faceting of rough gem material to Barion Cut by Basil and Marion Watermeyer produce an optically attractive gem is (c. 1970). Ta highly specialized and skilled art. In the case of coloured stones, whilst Over many years, several facet designs rarity, colour and weight are the claiming to enhance visual appearance and predominant factors determining the value, scintillation have been published. This is the overall appearance has considerable particularly so in the case of diamond, influence on the attractiveness of the stone, where apart from its colour and weight a and heart shaped, shield, navette, cushion, considerable proportion of the value of the mixed cut and many other geometrical stone is related to the geometry and correct designs have appeared. © Gemmological Association and Gem Testing Laboratory of Great Britain ISSN: 1355-4565 Figure 1: 42 et (Nd) YAG 'PinwheeV design (Ed Hume 1997) cut by Doug Morgan 1997 showing 'wheel' in the bottom of the stone. Although there are ideal optimum cutting angles for the pavilion facets so that Figure 2: 'Canadian Maple Leaf design cut by total internal reflection of the refracted rays Al Manastar 1999. will take place and give brilliance and dispersion, depending upon the refractive caricature face is polished into this index of the gem material, commercial background (Figure 3). Another novel effect considerations often override this ideal in is produced by cutting the crown as a order that maximum yield in weight and domed hemisphere on which a pattern of size can be obtained. From the jewellery small flat facets are placed, as in the ' i'Jk^jMÊ-".' 193 design aspect, often stones are cut with a 'Dandelion' cut. This was designed by the shallow pavilion for mounting in finger Finnish cutter Tauno Paronen, an193d a rings. This can result in low refractive index photograph of a stone cut by the author in stones having a 'fish eye' where light rays quartz to the diagram(1) by W.J. Maloney Jr. incident to the table pass straight through is shown in Figure 4. the stone without contributing to the brilliance and dispersion. Cutting for special effects Many novel effects have been produced however by deliberately cutting selected pavilion facets at angles below the critical ideal, for example the 'Pinwheel' design by Ed Hume (1997) which shows the 'wheel' as a dark pattern in the bottom of the stone (Figure 1). Other novel effects have been produced by frosting certain facets such as in the Canadian Maple Leaf Cut by Al Manastar (1999). In this some of the pavilion facets in the design of the maple leaf are frosted and so stand out from the remaining polished pavilion facets (Figure 2). This frosting technique has also been used to amusing effect as in the faces by Art Grant where a domed crown is frosted and a Figure 3: 'Funny face design cut by Art Grant. J. Gemm., 2002, 28, 4,193-209 Figure 4: 'Dandelion' design by Tauno Paronen Figure 5; Early groove cut 3.33 ct amethyst. and W.J. Moloney Jr. cut by Doug Morgan 2002. In 1992, computer aided design (CAD) was applied to faceting by Robert W. Strickland, and his program GemCad 4.0 became available for amateur use. The program enables one to draw a gemstone pattern directly on the computer screen, it then calculates all the necessary cutting 195193 angles, shows the light ray paths and prints an expected view and brilliance factor for that particular design. A skilled user of this program can thus calculate the best cutting data for any rough material without wasting time and material. A data base (DATAVUE) of over 3500 patterns having standard flat facets is also available, and it is apparent that Figure 6: Early groove cut 35.44 ct citrine. facet design is now on a sound mathematical foundation. The formation of lapidary societies, guilds and clubs in America, Australia and more recently in this country, has been instrumental in bringing together the large number of individual cutters with their design contributions, and there is a continual forward movement to invent and introduce new and novel facet designs, as shown in the regular contributions to the American Lapidary Journal and various club and guild newsletters. In recent years considerable interest has been shown in the cutting of concave facets on gemstones as a design feature both of novelty value and to increase the attractiveness of the stone. Figure 7: Early groove cut 17.58 ct rock crystal. Development of concave faceting of gemstones Gonard was granted US Patent 946,939 for a method of cutting concave crown facets on diamonds "with the object of securing increased brilliancy" and a 2.68 ct old mine cut with a concave table is shown in Gems & Gemology Gem Trade Laboratory Notes, p.171-2,1989. It is believed that this diamond may be related to this patent. These are said to be the only three diamonds with concave facets ever seen in the GIA Gem Trade Laboratory. With regard to coloured fancy stones, the 'Freehand Sandstone Faceters' in the area around Idar-Oberstein over 100 years ago were producing stones with concave cut Figure 8: 7.5 ct smoky quartz 'Daisy' cut 1960 facets on the pavilion. The sandstones were design. over 100 cm diameter and had a 20 cm face Cutting of concave facets with profiles to give concave grooves in the stones. These became quite fashionable Concave facets have been cut on around 1920, and later in 1950-1965, some diamonds, and two small triangular synthetic stones were cut using metal lap diamonds with spherically concave crown wheels^. Typical examples of these stones facets are reported in Gem Trade Laboratory are shown in Figures 5, 6 and 7.