For a cube with side length \( s \), the diagonal \( d \) can be found using the Pythagorean theorem in three dimensions:
\[
d = \sqrt{s^2 + s^2 + s^2} = \sqrt{3s^2} = s\sqrt{3}
\]
Substituting \( s = 6 \, \text{cm} \):
\[
d = 6\sqrt{3} \, \text{cm}
\]
Thus, the diagonal of the cube is \( 6\sqrt{3} \, \text{cm} \).