Many minerals can be separated readily in one direction by simply
making a small indentation with a harder mineral, then introducing the
blade of a knife into the scratch and striking it a sharp blow,—this
separates the crystal. There are certain planes at right angles where
the crystal can be separated; this property is called cleavage and the
planes, cleavage planes.
In some minerals cleavage is difficult to produce, while in others such
as mica and rock-salt, cleavage is highly perfect and the number of
separations produced is only limited by the thickness of the blade used
in separating the planes.
The property of cleavage is very useful and of great assistance to the
lapidary, as it enables him to shape a diamond or other hard stone
nearly to the size he desires, and at the same time to save the extra
material cleaved off, which can be used for smaller gems, and which
under other conditions would have to be ground away.