[widgitmaster]

Use diamond wheels for carbides, plastics and synthetics.
Use CBN for most steels, including tool steel and HSS.

CBN and Borazon are the same grit type. It is a man-made material.
Most diamonds used in manfacturing are man-made synthetic diamonds.

The basic abrasive types are aluminum oxide, silicon carbide, cubic boron nitride (cBN) and diamond. Aluminum oxide is the all-purpose abrasive with the widest variety of applications. Due to its inherent sharp shape, silicon carbide will be used to grind aluminum, magnesium and titanium alloys, as well as polymeric materials and rubber. Silicon carbide works well on hard materials; however, diamond is a better abrasive for grinding hard and ultra-hard materials like carbides, glass and ceramics. Diamond is carbon and has a chemical affinity for iron - resulting in heavy wheel wear and poor performance when machining ferrous materials. Diamond is always used on non-ferrous materials.
Cubic Boron Nitride (cBN) has been available since 1969. It is an extremely hard and abrasion-resistant material. It can be used to great advantage in the machining of ferrous materials. It is significantly harder than aluminum oxide, but moreover it is a conductor of heat. Aluminum oxide and silicon carbide - the conventional abrasives - are refractory materials and act as insulators. cBN and diamond - the two superabrasives - are conductors of heat. Diamond conducts heat by a factor of six times over that of copper. These superabrasives will always inherently grind cooler than the conventional abrasives.
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Metal Bonded Diamond Tools are "impregnated" with diamonds. This means that selected diamonds are mixed and sintered with specific metal alloys (such as iron, cobalt, nickel, bronze, and over 200 other components in various combinations). To achieve the best cutting performance possible on any materials such as sapphire, glass, granite, tile and etc. The metal bond surrounding the diamonds must wear away to continuously keep re-exposing the diamonds for the diamond tool to continue cutting. Diamond is the hardest material known to man kind. When used on tools, diamond grinds away material on micro (nano) level.
If the tool becomes overheated, the metal bond does not wear away, instead it "glazes over" the diamond. Hence coating or covering the diamond. The metal bond then becomes the cutting agent rather than the diamond. Generating more heat. The diamond tool will eventually chip the material or break it, due to excessive heat build up in material. Same heat may damage the diamond tool itself, by causing heat cracks in the diamond section.