referring now to the drawings in detail , fig1 to 3 illustrate a dressing tool 1 for a grinding wheel 2 . the dressing tool is designed in the preferred embodiment as a dressing slab . the tool 1 is provided with a holder 3 which carries a diamond plate 4 . the diamond plate 4 is formed of diamond grains 5 of the same grain size . diamond grains 5 are arranged in such a way that they are in direct contact with adjacent diamond grains 5 . for holding the grains , an electro - plating bond 6 made of nickel or cobalt is provided . individual diamond grains 5 , of which one is shown roughened , especially by etching with a metal under the application of heat . as shown in fig4 the surfaces of the individual diamond grain in the shape of a cubic octahedron are provided with numerous pores 7 which have the shape of indentations with undercuts as clearly seen in fig5 . as a result , the surface area which is active for holding the diamond grain within the bond is enlarged by a factor of at least two as compared with the natural surface size and , upon electroplating , the metal is able to penetrate in a root - like fashion into the individual pores , so that holding or adhesion is substantially improved . it is thus possible to arrange individual diamond grains in a high concentration when electroplating bonding agents are used , and to enhance the performance capacity of the dressing tool . this applies not only to slab - like dressing tools , but also to dressing tools designed in the form of rolls or wheels . the present invention is not limited to the arrangement of diamond grains in one layer . fig6 shows a further embodiment , in which a multiplicity of diamonds can be arranged in a layerless structure wherein individual diamonds or diamond grains are in contact with diamond grains lying alongside , above as well as below . the use of diamond grain sizes of different orders is possible in accordance with fig7 where small diamonds are located in the gaps between the larger diamonds ; this arrangement permits a further increase in the diamond content . the diamonds utilized in the embodiments described are synthetic diamonds , which are particularly suitable for use in tools according to the invention . however , this does not exclude a use of natural diamonds . as shown in fig8 and 9 , one embodiment of the invention provides for the arrangement , on a diamond layer 4 , of a wear protection layer 10 which preferably has the thickness of 0 . 1 to 1 mm and consists of diamonds which are bonded in an electroplating metal such as cobalt or nickel . the surfaces of these diamonds in the wear protection layer 10 are again preferably enlarged by etching . the provision of protection layers of sintered materials is known from other fields of application . in those cases , the protection layers are produced by powder - metallurgical processes . this involves the disadvantage that , in order to obtain a uniform layer thickness in the outer protection region , the thickness of the protection layer cannot be below a relatively large value , since even thicknesses of 0 . 8 mm cause problems in powder metallurgy . a further disadvantage of conventional methods is that , in powder - metallurgical production , the diamond concentration has a strict upper limit for process engineering reasons , and a concentration of more than 60 or 2 . 6 carat / cubic centimeter has not hitherto been feasible in practice . these disadvantages of the powder - metallurgical methods can be avoided by using electroplating , for example the electroplating of metals such as cobalt and nickel . such electroplating allows a precise limitation of the thickness of the lateral protection layer so that , for example , layer thicknesses of the range of 0 . 2 to 1 mm can be used . it is then possible , especially for lateral protection to increase the diamond concentration substantially , namely to a concentration of 150 to 200 , which is equivalent to 6 . 6 to 8 . 8 carat / cubic centimeter . synthetic diamonds and also natural diamond grains can be used for this , whereby a substantial improvement in the holding of the diamond grains within the electroplated layer is generally obtained when the diamonds show an enlargement of their surface to preferably at least twice its natural size , obtained especially by etching , which would not lead to significant advantages in the case of a bond produced only by powder - metallurgical means . a special advantage here resides in that particularly small grain sizes can be used , which are only about half conventional grain sizes . this ensures extremely firm seating of the superficially pretreated diamonds in an electro - plating bond , so that the utilization level of the expensive diamond material is improved . if the wear protective layer 10 is provided on the front and back sides of the diamond layer and additionally also on two other sides , the diamond layer 5 , 6 is protected against movements in all directions . in fig9 and 10 , a dressing slab is illustrated which has diamond grains 5 arranged in one layer . these diamond grains are artificially roughened and bonded in a metal 6 by electro - plating . to protect diamond grains 5 , two protective layers 10 and 12 are provided , the thickness of which approximately corresponds to the thickness of the diamond layer 4 , 5 . the grain size of the diamond grains 5 is about 750 μm . therefore the protective layers 10 and 12 are also of a corresponding thickness . the protective layers however consist of diamond grains of substantially smaller size , in particular of grains or the order of 70 μm , size , for example . the additional protective layers 10 and 12 prevent lateral &# 34 ; washing - out &# 34 ; of the bond of the effective diamond grains 5 . this results in the advantage that individual diamond grains 5 of the dressing tool can be utilized to a higher degree , because they are firmly retained by the protective layers of the both sides of the diamond layer for a longer period . this is true in particular after a partial consumption of the protective layers according to fig1 , that is to say a state in which individual diamonds 5 protrude outwards i the feed direction , corresponding to the arrow , but are protected from lateral breaking - out by the protective layers 10 and 12 . the result of the provision of protective layers 10 and 12 is thus an improvement in the holding of the diamond grains arranged in the middle . the holding is anyway improved over comparable known arrangements by the artificial roughening of their surfaces and their bonding by electro - plating in an arrangement , in which they are in direct mutual contact . the thickness of the diamond coat effects the precision of a dressing operation . for this reason , dressing slabs with a diamond coat thickness of not more than about 1 mm are particularly suitable . a diamond grain size of for example d 711 is suitable for this purpose . in the case of multi - layer diamond surfaces , smaller diamond grain sizes , for example d 501 , d 301 or d 181 , can be used , maintaining the densest grain arrangement possible , in which a large proportion of adjacent diamond grains are in mutual contact . in further modification of the dressing tools according to the invention , diamond grain mixtures of different grain sizes are used , for example d 711 with d 501 or with d 181 or with d 46 or mixtures of several of these grain sizes , for increasing the density of the diamond grain arrangement . three examples a , b and c of different types of dressing slabs are presented below . of the three examples , design a corresponds to the known structure , example b shows the results obtained with a slab which has a high diamond proportion of 0 . 8 carat , but without an artificially enlarged surface as in example c which has the same diamond proportion as design b , but with the surface enlarged according to the invention . in all cases , the dressing tools are slabs with a coat area of 10 mm × 15 mm and a working edge length of 10 mm , and with a diamond coat of a layer of diamons grains . the results were obtained when dressing corundum grinding wheels of a diameter d = 500 mm and a width b of 33 mm , the dressing being taken to a diameter of 300 mm . the dressing experiments were continued until 10 mm of the 15 mm deep grinding coat of the dressing slabs had been worn off . the table which follows shows the volumes removed by the dressing from the grinding wheels . ______________________________________designs ofdressing slabs a b c______________________________________diamond grain size d 711 d 711 d 711diamond grade original original special topograph as a result of enlarged surfacediamond content 0 . 45 ct 0 . 8 ct 0 . 8 ctmetal bonding in sintered electro - electro - the diamond coat metal plated plated ni bond ni bondgrinding wheel 6 . 5 dm . sup . 3 14 . 0 dm . sup . 3 21 . 1 dm . sup . 3volume removedspecific removal 14 dm . sup . 3 / ct 17 . 5 dm . sup . 3 / ct 264 . dm . sup . 3 / ctfrom the grindingwheels , referredto 1 ct of diamond______________________________________ it will be understood that each of the elements described above , or two or more together , may also find a useful application in other types of dressing tools for grinding wheels differing from the types described above . while the invention has been illustrated and described as embodied in a dressing tool for grinding wheels , it is not intended to be limited to the details shown , since various modifications and structural changes may be made without departing in any way from the spirit of the present invention . without further analysis , the foregoing will so fully reveal the gist of the present invention that others can , by applying current knowledge , readily adapt it for various applications without omitting features that , from the standpoint of prior art , fairly constitute essential characteristics of the generic or specific aspects of this invention .