Typically a brittle material, such as glass, is used as a grinding stone, also referred to as the lap. Sometimes the lap is of the same material as the workpiece. By using brittle materials for the lap conchoidal fractures occurs. This means that they break randomly without any natural planes of separation, like when glass shatters.
There are two different types of lapping; firstly it can be performed to gain a flat surface. This can be compared using a polishing cloth on a car. Secondly, it can be used to mate two surfaces. To mate two surfaces, they are rubbed against each other with an abrasive inbetween. Then, the surfaces will adapt to each other; however, the surface may not necessarily be flat.
Lapping results in very fine tolerances and is measured in flatness and roughness.
Picture: Slack & Parr Ltd