There are two types of abrasives indeed called natural and artificial. Talking about the natural abrasives it occurs as minerals or rocks in the crust of the earth. Natural Abrasives also holds amazing examples such as diamond, corundum and garnet and quartz.
On the other hand, there is a artificial or synthetic abrasive group called a number of materials having high hardness. It is all about aluminum oxide, gl*** fall and carborundum regarded of this group.
Talking about the forms of abrasives, they are used in the context of both blocks from an powdered form. The blocks form are indeed pieces cut having suitable shapes gain from the abrasives material. Moreover, they are also used in millstone, cutting edges, whetstones and grindstones.