GB2144622A - Gemstones and apparatus for their lapidation - Google Patents

Abstract

Apparatus for gem lapidation comprises a curved abrasive surface (S2) and a faceter (A-C). The abrasive surface (S2) is rotatable relatively to the faceter. The faceter is mounted at two points (A, B) on another spherical surface (S1) such that in use when a gem is held by a gem- carrying terminal (C) of the faceter against the abrasive surface (S2) subsequent movement of the faceter relatively to the abrasive surface causes the gem to move along a path having substantially the same radius of curvature as the abrasive surface whereby a constant angle is maintained between the gem carrying terminal and the normal at the point of contact of the gem with the abrasive surface (52). Gemstones having at least some spherical facets (convex or concave) may be so made. <IMAGE>

Description

SPECIFICATION Gemstones and apparatus for their lapidation Forcentries, gem lapidation has been carried out cutting rough crystals with various sets of flat facets, irrespective of the final shape required. With thiscritenum of construction, a conventionally-cut gem is reduced to an optical system consisting only of prisms and flat reflective surfaces. For a light source, this optical system always produces a virtual image; moreover, given the reduced dimensions of a gem, the image produced is stongly strangled from the stone's structure; which is why it is received by the eyetransitorily and intermittently, due to the continuous movements between the light-source, gem and observer.

In accordance with one aspect ofthe present invention, apparatusfor gem lapidation comprises a curved abrasive surface and a faceter, the surface and the faceter being relatively rotatable, and wherein the faceter is mounted such that in use when a gem is held bythefaceteragainstthe abrasive surface any subsequent movement ofthe faceter relatively to the surface causes the gem to move along a path having substantially the same radius of curvature as the abrasive surface whereby a constant angle is maintained between an arm ofthefacetercarrying the gem and a normal to the point of contact between the gem and the abrasive surface.

In accordance with a second aspect of the present invention, a gemstone presents at least some spher ical facets. The use of spherical facets, whether they be concave or convex, in the place of traditional ones, transforms the gem into a true and proper catoptric system capable of a greater dispersive effect, producing images closerto the stone which, however, appear more luminous to the observer since they are closet to and less strangled from the stone's structure.

Some examples of gemstones and apparatus for their lapidation will now be described with reference to the accompanying drawings, in which: Figures 1 to 4 illustrate gem cuts respectively in a conventional manner, with concave surfaces, convex surfaces, and partially concave ones; Fig. 5 is a diagram of a photometric experiment; Figs. 6A and 6B each show respectively a first apparatus, sectionally and in a plan, for carrying out the cutting of gems according to the presentinven- tion in the case of concave facets; Figs. 7A and 7B are analogous to Figs. 6A and 6B, but they shown an apparatus designed for cutting gems with convex surfaces; Fig. 8 is a diagram of a conventional tripodal facetting utensil;; Figs. 9A and 9B are lateral views of a different apparatus forthe present invention, where one is rotated through 900 with respect to the other; Fig. 10 is the view of a similar apparatus to the one shown in Figs. 9A and 9B, but used in the facetting of a gem with convex surfaces; Figs. 11 and 12 are respectively lateral and above views of another apparatus; Figs. 13 and 14aretwo graphs showing the luminous efficiency of a gem which is respectively conventional, and with spherical facets.

Thatthis catoptric system bringsthe image of a light-source closertothe physical structure of a gem may be verified by applying the equations below, which link the conjugate points sand s'to a dioptric of radius R,which separates the media having refractive indices n and n'(a) and the conjugate points sands' with respect to the reflective sphere also of radius R (b), as shown below::

In fact, applying the equations (a) and (b) succes sivelytothe profiles of Figs. 1,2 and 3 in table 1 ,that is,to gems having, respectively, flat, concave, and convexfacets, butwith, however, identical shape, weight and refractive index, one obtains the results shown on the synoptic table below: COMPARATIVE TABLE Position ofconjugate points with toa gam and conjugate points with respect to a ici1 faceted gem, as shown in in the diagrams of figs. 1, 2, 3 fcc kidl: n 1.70, R= 75 cm and the incident ray is to toupperpartofgan,cc "crown".

j Postulates of Flat-faceted Concave-faceted Convex-faceted operation gems gems gems Distance Distance Distance Distance Distance Distance Distance Distance Gem- Gem- image- Image- image- image- image- image- Eye I Eye w Gem Eye Gem ; Eye I Gem Eye I (cm) (cm) (cm) (cm) (cm) (cm) (cm) (cm) 200 200 , 200 -200(") -300(") 46(") 54(") -37(") @31(") # 7 100 -# (R; H) i7i) 63( l ~37(ì 137() 100 -(") 37() jti": ~ ~ ~~~~~~~~~~ ~~~~~ ~~~~ 63(") -37() 037(") 200 100 -200 300 -8,7 108.7 9,5 90,4 # 100 # # -9.1 109.1 9.1 90.5 1) Distanceof images obtained by sbplifidcalculations, usingtheformula Ce) and(b), withoutring influence of refractive index or ofdistance between facets when calculatingopticalpaths.

2) Valuesmarked byan asterisk(") refer toimages produced by reflextions on external facets.

3) Thesign(+)indicatesreal images abovethegem;thesign (-) indicates virtual images beiO"' tin gen.

These data relate to the conditions of use typical in the presentation of a gem. For all the distances provided for by the light-source, the concave-faceted gem always produce virtual images that are very close to the bottom ofthe stone and, therefore, turn outto be less strangled from the stone's structure. At the same time, they appear as a result even more luminous to the eye of an observer, in thatthey are closertothe latter.

However, in the case of convex-faceted gems, the real images obtained are not seen by an observer positioned one metre awayfrom the object, because these images reformed in the air, over the stone. In recompense, the same images, as it has been shown in practice, are seen in all their splendour by a distant observer positioned more than 5 metreawayfrom the gem. In fact at such a distance, the eye ofthe observer, as it directs itself towards the gem, is already adjusted to an infinite vision, where he will see, superimposed upon the gem's contours, either the image ofthe source provided by the external surface reflection, or the image produced by a total internal reflection.This is the typical case for a gem displayed on the edge of a box in a theatre, illuminated byten orso hanging lamps, and which is observed from a distance by spectators standing in other boxes, or in the pit.

Atthis point, having verified the geometric effects of bringing these images closer,the supposed increase in luminosity of the images caused by the use of spherical facets now remains to be proven and measured, as well as the increase in the quantity of light picked up by the eye of an observer.

With this aim in mind, photometric tests have also been carried out in the laboratory, according to the diagram in Fig. 5. Here, one sees that the projector lamp I, by means ofthe silvered mirror 2, sends a beam of light perpendiculartothe "table" orflat surface of a gem 3 rotating on its axis of symmetry 32; the flashes ofthe total reflections sent back from the gem, cut into the photoelectric cell 4, which is positioned obliquely to this axis, at a distance of one metre, which sensitisesthe recording apparatus5 with its pulses. Using this photometric apparatus, various series ofcomparativetests have been carried out in a University Laboratory of a high standard, on two colourless beryls, having identical form and dimensions (approx. 20 carats) with flat rectangular tops, and cut like an emerald.One ofthe gems had conventional flatfacets and the other had concave spherical facets. Under identical experimental conditions (intensityofthe incident light; number of rotations per minute; distances between light-source - gem - photoelectric cell; speed of slip of recording sheet, etc.) the recording-apparatus has provided the graphs reproduced in Figs. 13 and 14. These present respectively the impulses caused by the reflections of theflat-faceted gem and those caused bythe concave-spherical-faceted gem, during rotation on their axes of symmetry 32.

The following notes aretowards an interpretation of these graphs: a-The graphs repeatthemselves continually at each full rotation ofthe gem; a foreseeable fact given that the gem's parameters will always be constant during the tests; b-The height ofthe curve peaks indicate the maximum luminosityobtainedbyeachsingleflash, i.e. by each total reflexion produced by the rotating gem and picked up bythecell; c-The total surface which is delimited at each rotation bythe upper contour of the graph and by the horizontal base-line (see the shaded area on graphs) indicates the total quantity of light reflected from the gem and picked up bythegem during afull rotation;; d-The number of peaks occuring in a cycle indicates the number of flashes, orthe number of total reflections occurring in the gem in the course of afull rotation and picked up bythecell.

A simple visual comparison of the two sets of graphs permits one to affirm that, excluding any error of calculation or subjective observation, the use of spherical facets substantially increases the value of all the parameters of the luminous outputofa gem, as specified in the paragraphs b-c- d-.

Unfortunately, the photometric tests have not been completed with angle measurementsforalsode- terming the dramatic increase ofchromatic dispersion as seen by the naked eye. Nonetheless, it seems reasonable to believe that the lenticular effect ofthe diopters of entrance and exit from the gem, neces sarilyconstitutes an increase in the lateral chromatic dispersion usually produced by conventional flatfaceted gems.Furthermore, it must be remembered that in a catoptric system constituted by one spherical-faceted gem, an axial componentofthechromatic dispersion, inexistent in flat-faceted gems, is also automatically produced, and this superimposes itself upon the component already laterally increased by the diopters; thus a dual strengthening ofthe so-called "fire" ofthe gem takes place.

Finally, it should be notedthatthe progressive reduction ofthe radius of curvature in a sphericalfaceted gem promotes the intensification ofthe stone's total brilliance in the sense that, as the radius becomes smaller, the image from a light-source, whether real orvirtual, gets increasingly closerto the body ofthe stone; and this effect involves a progressive reduction in the diaphragm-openings ofthe reflected light-beams, benefitting the eventual optical output. Ofcourse, this faculty should not be exaggerated, otherwise the external appearance ofthe gem might become too different from that of the traditioned gem, with possible counter-productive effects on commericialization.All that remains to be said is that as one lowers the carat ofthe gem, one may have accordingly decreasing radii of curvature, since the gradual reduction ofthefacets, which will be accompanied buy a decrease in the gem's dimensions, results naturally in a spherical ball of progressively smallercamberand gradually less-accentuated edges. Evidently, only practice will establish whatthe minimum radius of curvature is to fit best the dimensions of a given stone. From these introductory notes the following may be concluded: 1 -Thatthe adoption of spherical facets actually increases all the factors of brilliance (external and internal brilliance; brilliancy ofthe sparkle, and brilliancy of dispersion), all ofwhich contribute to the total brilliancy of the gem; 2-That the concave facets are suitable for gems intended to be viewed by close observers; 3-Thatthe convex facets are recommended only for gems intended to be viewed mainly by distant observers; 4-That as a stone's dimensions decrease, the radius of curvature ofthe spherical facets may also be decreased, thus improving the luminous efficiency of the catopric system.

It is possibleto producegemswithflatfacetsinthe upper part or "crown", and spherical facets in the lower part, or "pavillon". This system may be adopted when one wishes to conceal the use of spherical facets, thus deliberately cutting out a part of the obtainable increase in brilliance. It is also possible to produce gems which possess at once concave, convexandflatfacets, placed together ion a group, or in alternation, in both the "crown" and the "pavillon", or just in the "pavillon". Also foreseen are gems principally intended to obtain new optico-ornmental effects, which may be contemplated by either distant or close observers, even if this results in fewer flashes being observed, for a given movement relative to the gem - light-source- observer.In fact, close observers will see the total reflections produced by the concave facets of the "pavillon"; whereas distant observers will only see those produced by the convex facets.

The notes which follow describe the kinematic principles which determine the cut of a spherical faceted gem, and a basic apparatus for this process.

However, for a better understanding ofthis, it is worth a brief reminder of what the essential process for the preparation of a normal flatfacet is. Briefly, this process involves rubbing the uncut stone against a rotating disc, normally of metal, so that the wearing-down resulting from the interference between the appropriate abrasives, conveniently scaled-down in their dimensions, will give, as a result, the dimensions and angles required forthe facets being processed, throughoutthe successive phases of rough-shaping, lapping and polishing. With the manufacture of spherical facets, the whole process is identical, but, obviously, the phases of rough-shap ing, lapping and polishing must be carried out by rubbing the rough stone on a sphere-shaped cover, or bowl, ratherthan on a flat disc.

The apparatus drawn in Figs. 6 and 7 of tables 5 enables this aim to be realized, respectivelyforthe production of concave spherical surfaces (Fig. 6) or convex ones (Fig. 7). Both diagrams are characterized in that they constitute two contiguous, coaxial and concentric sphere-shaped covers or bowls, with the same radius of curvature; the central one is rotating and the abrasion necessaryforthe production ofthe facet occurs on it; the second outermost one is fixed and serves as a surface to support two of the three support points of a conventional tripodal faceter (this fixture will be referred to as the "faceter" from now on, for purposes of brevityand twill consist, for example, of the Ptype faceter produced by the firm IMAMASHI Mfg. Co.Ltd. TokyoJP). During the operation, the two points of support in the faceterA and B, remain throughout in the external supporting sphere-shaped cover or bowl, S1,whilethe third C, which isthegem,will be placed in contact with the internal rotating cap S2, which contains the appropriate abrasives. One may observe that for all the possible variable positions given to the faceter, either in the search for a better direction of abrasion, or in order to place the stone in a zone ofthe most suitable velocityforthis abrasive process, the three ends of the faceter will always be in the same sphere ofwhich the respective sphere-shaped covers or bowls of supportfrom a part.More importantly,thefacet which isto be formed will have the same curvature as the abrasive cap, whilst the initial angle with respect to the perpendicular of the contact pointwill remain practically constant until the desired dimension for the facet in process is obtained. Whatfollows will be the known re-iteration ofthe operations seen now for all facets required from the selected cut, and this will be carried outwith the help of goniometers, which are provided on the faceter an which are represented by the diagrams in Fig. 8.Goniometer Ecauses the gem to rotate about its own axis 32; this rotation then brings the certain sections ofthe various facets into contact with the cutting-edge; goniometer Zcauses the gem to rotate about an axis perpendicular to the axis 32 of the gem and it serves to give to the same section of a whole set of facets the angel which they require for the shape of the gem.

This apparatus may be provided with a rectangular sector of the sphere-shaped cover S3which has the same radius of curvature as the other covers S1 and S2on which it mayslidefreely in all directions. The sector 53 has a longitudinal spline fit to accommo date the supportterminals A and Bofaconventional faceter; this faceter, with fulcrum atA and B, is free to rotate aboutthe axis ofthe spline until it allows contact between the gem-carrying terminal Cwith the abrasive cover S2.It is clearthat in this kinematic arrangement, the gem-carrying terminal Cwill always move on the sphere to which the covers S1 and S2belong, maintaining the initial angle which itis given with respect to these constant. The advantage ofthis accessory isthat it allows the gem being processed larger and more varied displacements upon the abrasive cover S2. without the already cumbersome cover S1 having to be increased in size forthis purpose, nor with the distance between the terminals A and B having to be altered.Of course, in ordertoavoid abrasion on the surface of contact S3, the rotating abrasive cover S2will have to be lowered by about one tenth of a millimetre with respect to the fixed cover Si, and thus, its radius of curvature will be reducedatthesametime, bythe same degree.

Figs. 9A and 9B of table 6 represent an application of these principles. In these, the supportterminals A and B, ratherthan leaning against the fixed cover are coupledwith ashaftHwhichclosesaforked arm K, pivoted on P, by means of a universal joint; the point Pis the center of the sphere to which the abrasive sphere-shaped cover S belongs, the latter's axis of rotation also passes through this centre. The faceter, which has its terminals of support A and Bfixed on the shaft H in a position symmetrical to the geometric centre of H, pulled by the oscillations ofthe arm Kwill therefore always move tangentially to a sphere of centre Pconstructed with the same centre as the abrasive "cover". By causing the terminals A and Bto rotate around H, one will be able to bring the gem-holding terminal Cinto contact with the abrasive "cover" Sand the angle of this with respect to the vertical of the point of contactwill remain constant whateverthe movement imposed on the oscillating arm K.The use of an apparatus constructed thus offers the following advantages: 1 ) The elimination of a heavyandvoluminousfixed sphere-shaped cover or bowl S1 from the apparatus illustrated in the diagrams in Fig. 6A, 6B, 7A, 7B.

2) Possibility of varying the curvature ofthe facets by adjusting the length and position ofthe oscillating arm K, and substituting the one abrasive sphereshaped cover or bowl with another having the desired radius of curvature.

3) Possibility of producing convex-faceted gems by vertically suspending the oscillating arm Kabove a concave abrasive sphere-shaped cover or bowl, (see Fig. 10-table 6).

4) Possibility of setting up the apparatus horizontal- ly so as to permit a better view of the operation, a particularly useful position in the case of a re lapidationofa previouslyoutstone, orof onethat has been cutfaultily.

5) Possibility of limiting the displacements of the stone upon the sphere-shaped cover or bowl, in order to prevent the stone from falling off the edge of the cover, by simple adjusting ofthe position of ring L.

6) Possibility of mechanizing the displacements of the stone processed, by acting on the oscillating arm Kwith conventional automatic artifices (eccentrically rotating pivots etc.).

The Figs. 11 and 12 in table 7 illustrate another apparatusforobtaining the cut of spherical facets, respecting the basic need to keep the angle ofthe stone being processed constant with respect to the sphere-shaped cover or bowl, for whatevertranslation imposed on the gem-carrying arm. This involves a mechanical device constituting 3 sphere-shaped covers or bowls N1, N2, and N3, of equal radius of curvature; N1 isthe rotating abrasive sphere; N3 is the sphere of identical dimensions and coplanarto N1, but fixed, serving as a support to the oscillating cover N3 on which a gem-holding arm K1 is inserted, provided with conventional goniometers E and Zand freeto rotate about a pivot 0, lying in asingle meridian of N3. With this device one mayverifythat, placing Oonthe axis of S3and giving arm K? a length equal to the distance between the axis of N1 and N2, the gem will be ableto settle on the sphere-shaped abrasive cover, or bowl, maintaining the initial angle received at a constant. irrespective of the position or movement of N3 on N2: a requirement which, as has been stated, is indispensableforthe cutting of a spherical facet of predetermined angle with respect to the axis of symmetry ofthe gem.

Claims (15)

1. Apparatusforgem lapidation comprising a curved abrasive surface and a faceter, the surface and the faceter being relatively rotatable, and wherein the faceter is mounted such that in use when a gem is held by the faceter againstthe abrasive surface any subsequent movement ofthefaceter relativelyto the surface causes the gem to move along a path having substantially the same radius of curvature as the abrasive surfacewherebyaconstantangle is maintained between an arm ofthefacetercarrying the gem and a normal to the point of contact between the gem and the abrasive surface.

2. Apparatus according to claim, the apparatus comprising two contiguous at least part spherical surfaces, the surfaces being coaxial, concentric and of substantially equal radius of curvature, wherein one surface constitutes the abrasive surface and rotates in use, whilst the othersurface is fixed to a work-surface, and wherein support terminals ofthe faceter are carried by the other surface to carry out stably any displacement on the other surface, whilst a gem-carrying terminal of the faceter supports a stone being processed against the rotating abrasivesurface, thus ensuring th roughoutthe operation a constant angle between the gem-carryingterminal and the normal atthe point of contact of the stone with the sphere of which the two su rfacesform a part, whatever the position or movement given to the faceter.

3. Apparatus according to claim 2, further com- prising a rectangular sector of a spherical cover, having substantiallythe same radius of curvature as the at least part spherical surfaces on which this slides freely in all directions, the sector being provided with a longitudinal spline to house the supportterminals of the faceter, wherein the faceter is able to rotate, about a fulcrum defined by the axis of the spline until it permits contact of a stone supported by the gem-carrying terminal with the abrasive surface, the abrasive surface being positioned with a radiusofcurvature aboutonetenth of a millimetre less than the one surface.

4. Apparatus according to claim 1, the apparatus comprising a forked arm pivoted at a radially inner end bya universal joint placed on the axis ofthe spherical abrasive surface and at the centre of the spherical abrasive surface, the radially outer end of the forked arm terminating in a shaftwhich, in accordance with movements imposed on the forked arm, will always move tangentially to a sphere having the same centre as the abrasive surface, support terminals ofthefaceter being hinged on the shaft, positioned symmetrically aboutthe centre ofthe shaft, and being free to rotate about the shaft in such a wayasto allow a gem-carrying terminal ofthefaceter to lay a stone being processed on the abrasive surface to cut a facet having predetermined anglewith respect to the axis of symmetry ofthe gem.

5. Apparatus according to claim 4, wherein the curvature offacets may be determined by one or more of selecting an abrasive surface having a suitable radius of curvature, displacingthe original centre of oscillation to a distance from the vertical equal to the new radius employed, and altering the length oftheoscillating arm, these displacements and adjustments being enabled by suitably positioning axle shafts of the universal joint between the respective couplings.

6. Apparatusaccordingtoclaim4orclaim 5, wherein the oscillating arm is suspended above a concave spherical abrasive surface.

7. Apparatus according to claim 1,the apparatus comprising a first rotatable abrasive spherical sur face; a second spherical surface spaced from and fixed relativelyto the first surface,thefirst and second surfaces having substantially the same radii of curvature; and an oscillating cover having a spherical base supported on the second surface, the cover supporting a gem-carrying arm, the arm being free to rotate azimuthally about a pivot lying in a single meridian ofthe cover, and the arm having a length substantially equal to the distance between parallel axes of the first and second surfaces, so that a gem supported by a gem carrying arm may lie on the abrasive surface maintaining an initial angle with it constantwhateverthe position or movement of the first and second surfaces.

8. Apparatus for gem lapidation substantially as hereinbefore described with reference to any of the examples illustrated in Figures 6 to 12 ofthe accompanying drawings.

9. A gemstone lapidated by apparatus according to anyofthe preceding claims.

10. A gemstone presenting at least some spheric- al facets.

11. Agemstoneaccordingtoclaim 10, having at least some concave facets.

12. Agemstone according to claim 10 or claim 11 having at least some convexfacets.

13. Agemstone according to any of claims lotto 12, possessing facets having less pronounced curvature in the upper part, or"crown", so asto be able to minimise eventual loss in weight orto reduce difference in external appearance ofthe gem compared to the traditional gem's aspect.

14. A gemstone according to any of claims 1 Oto 13, possessing flat facets, particularly in the upper part or "crown" and spherical facets, concave and/or convex, in the lower part, or"pavillon".

15. A gemstone substantially as hereinbefore described with reference to Figures 1 to4,and Figures 13 and l4oftheaccompanying drawings.