Since it was discovered, the cutting technique of precious stones has changed very little. And yet, whether flat, concave or convex, the disk of the traditional buff wheel may not have given away all its secrets...

The buff wheel first used to be in silica, then in carborundum, but there is a great variety of them, from the small buff wheel measuring a few centimeters to the buff wheel which has a diameter over two meters, used in the water cutting workshops in ldar-Oberstein (Germany) since the 18th century. In the 1950s, cutting colored stone is becoming a passion for the gem lover.
This new market boosts the rapid expansion of the American, German and Japanese manufacturers who offer different machine types - small models for the amateurs, as well as stronger models for the cutting industry - but they are all built according to the system of flat, diamond-tipped disk or coated with abrasive powder.
There are not automatically any, if we refer to different optical surveys: for some, the results of cutting on concave or convex disks would indeed give more shine, more light to the stone. This technique, known as OMF (Optical Magnified Facets) in the United States (Lapidary Journal) was patented in Belgium in 1984 and in Italy in 1982, without meeting the expected success. And yet, according to its inventor Giovani Colliva, the dispersion effects is said to be intensified and to transorm the gem into a genuinen “catadioptric” system. The transformation of the traditional lapidating buff wheel into a convex buff wheel is rather simple since the cutting arm remains identical; the traditional flat disk, moved by a vertical axis, is being replaced by a horizontal axis and cylinder of different diameters, whether diamond-tipped or not. The curves are different then and the facets may be cut, one more concave than the other. The buff wheels,used for the cutting of cabochons may even, in certain cases, be used by the DIY fan as a cutting disk. The diamond-tipped cylinders are available in different granulations, as for the traditional diamond-tipped disks.
For instance: between 600 and 1,200 mesh (0.5 micron = 50,000 mesh, 1 micron = 14,000 mesh, 50 microns = 325 mesh). It may, furthermore, be expected that the tool manufacturers will soon bring out disks with more than 15,000 mesh. However, if the manufacturers have already made available to the lapidaries different cylinders for the concave cutting, the same does not go for the convex cutting as manufacturing those is more complex and requires the creation of different calibrating diamond-tipped gutters. It is a new market, widely open to the lapidating tool manufacturers since the range is very much extended indeed… It is advised, for the preliminary shapping, to work on a flat disk: indeed, during this stage, what has to be done is taking away as much material as possible.
Only then is it possible to start with cylindrical buff wheels for the actual cutting and polishing. The other advantage of the flat disk is in the availability of a large range of different granulation, from 60 to 15,000 mesh, an alloy of aluminium copper and resins.
Another important factor of the art of cutting is the cooling process: during the pre-shaping stage, more liquid is being used (water or cooling fluid like, for instance, a petrol based solution), wich is not as easy with a cylinder. During cutting, but also during the pre-polishing stage, the range of cylinders is relatively restricted, apart from the 1,200 mesh diamond-tipped cylinders. The phenolic cylinders have to be coated with diamond-tipped powder of different granulations. As regards the latest polishing stage, there are phenolic or polycarbonate cylinders, coated with diamond-tipped powder (the choice ranges up to 50,000 mesh) or oxides (cerium, tin, aluminum, etc.). The polishing coatings require less cooling liquids, moistening being more than sufficient. The Imahashi supplier has made available to the lapidaries, and that for a long time now, disks filled with polishing powders where there is no need for anything else to be added, except for the traditional cooling liquid. The American manufacturer, Polymetric Instruments, has launched on the market transformation units making it possible for the lapidary to keep his/her equipments: Grayes, Ultra Tee, Lee, Fac-ette, Raytech and even Imahashi who is working with free pliers (just like the diamond cutter), and so being different from the traditional system of the lapidaries using the support-column. The advantage of this transformation on the same basis resides in the fact that the stone will be lodged, after the pre-shaping stage, on the flat disk, exactly in the same place of the facet on the cutting cylinder.
Through visual comparisons, it may be noticed that the spherical facets increase the brilliance of the stone. It may be added that the concave facets are appropriate for those gems intended to be seen by close observers whereas the convex facets give better results when they are seen from a distance.
The combination of the plane, concave and convex facets when the table, the crown and the pavilion are being cut, enables the lapidary to create different optical effects: The convex surfaces make it look bigger and concentrate the rays of the light, sending off light spot like effects whereas the concave surface diffuse the light and bring more brilliance and fire. Take the case of the crown, the concave facets will send light effects in any and all directions while the convex facets will tend to increase the inner characteristics of the stone such as, for instance, the colour. The case of the pavilion is different since the concave look will seem convex seen from the top of the stone. Using the concave facets gives the impression that the stones have a higher refraction index. So, quartz may look like beryl, beryl like topaz, topaz like corundum and corundum like zirconium.
Another irreparable effect, the concave facets influence the colour of the gem which is intensified through the same optical effect. This influence on the central colour of the stone is more sensitive with weak colour gems, such as for instance, the aquamarines. During cutting, the critical angle nevertheless plays an important role. Once more, the concave facets are going to shake the established rules: thicker girdle will be possible and it will also be possible to cut in the weight of the gems. An argument which may awake the interest of the people who have shown the least interest ...
So it seems obvious that these techniques offer a multitude of possibilities to ‘play' with the light, the weight, the cut or also the shape of the stone. The whole art of the lapidary is indeed shaken by it Now we can expect heated discussions!
Up to now nobody has so far studied the possibility to cut the diamonds in concave or convex facets. But why would it not be possible? The material is there on the market for the cut of the girdles and the look for diamond-tipped disks or also an adaptation of the top (pliers’ support) and diamond-tipped cylinders, seem possible without too much investment. The Technological and Scientific Research Centre (WTOCD) of the HRD (Antwerp) has furthermore elaborated a brand new cutting technology based on diamond-tipped disks cooled by lubricant, which appears absolutely revolutionary. The use of cast iron cylinders, coated with diamond-tipped powder, is not to be excluded, provided they make use of a good binding. Indeed, following the faster rotation and the centrifuge force exercised, the oil coated powder or usual binding coated powder will tend to fly away quickly The advantage of this technique resides, on the one hand, in the use of only one diamond-tipped disk for the cutting and the polishing and, on the other hand, in the absence of a cooling system (to be seen in the near future).
The cutting mill would become more compact, handier. Some “cutting" issues still remain to be solved such as, for instance, the rotation speed of the cylinders: if the coloured stones are cut at a speed between 100 and 1,500 turns per minute, diamonds are cut at a speed reaching more than 3,500 turns.
The cutting machine for diamonds will therefore have to be far stronger than the one used for the cutting of coloured stones.
Another handicap: the positioning of the dop. If the lapidating of the coloured stones does not entail in cutting problems following the diamond-tipped disks, diamonds require on the other hand, the possibility to rotate the head of the dop. The outside, inside, drifting positions are the basic principle of diamond cutting.
The cutter regularly changes the position. In the case of concave facets, it will be possible to change the position of the dop during cutting. For this reason, it will only be possible to use a ‘healthy‘ rough diamond, without any inner distortions (neat). Always according to the WTOCD, the new diamond cutting technique would not be hindered by that problematic of crystalline structure. Consequently we are impatiently waiting for this technology which will revolutionize diamond cutting, discovered by Louis de Berquem, more than five centuries ago.
To conclude, the sawing experimenting goes along with the theory of the concave facets. Take the example of a lot of rough diamonds of +- 7/8 per carat, sawn with saws of a thickness of 6.5 to 7: the result will be a +- 5 to 5.5% loss with flat surfaces. During the sawing of the same lot with 4.5 to 5 thinner saws, the loss will be 1 4%. The surfaces will, however, often be concave and convex. During the cutting stage, the second lot may lose between 7 to 10% more than the first one. A more striking fact, the second lot containing concave and convex sawing sides will have more life and more brilliance than the first one. For this reason, a diamond dealer who is buying rough lots to sell them sawn, will be using the thinnest saws. The advantage is twofold: less loss and more ‘life‘! Based on the experiments already made on the coloured stones, the vivacity and the already legendary fire of the diamond will be increased by this new technique. An in-depth study, theoretical as well as practical, is really worth being carried out...