An abrasive tool having a working portion which includes an abrasive insert which is exposed to high temperatures during manufacture or use of the tool. Examples of such tools are dressing or turning tools and surface set or impregnated drill bits. The tools are characterized by the abrasive insert comprising a mass of diamond particles present in an amount of 80 to 90 percent by volume of the insert and a second phase present in an amount of 10 to 20 percent by volume of the insert, the mass of diamond particles containing substantial diamond-to-diamond bonding to form a coherent skeletal mass and a second phase consisting essentially of silicon, the silicon being in the form of silicon and/or silicon carbide. these abrasive inserts have been found to have substantial strength due, at least in part, to the diamond-to-diamond bonding and to be capable of withstanding temperatures of 1200.degree. C. under a vacuum 10.sup.-4 Torr or better or insert or reducing atmosphere without significant graphitization of the diamond occurring.