  The orbital magnetic moment of a finite piece of matter is expressed in terms
of the one-body density matrix as a simple trace. We address a macroscopic
system, insulating in the bulk, and we show that its orbital moment is the sum
of a bulk term and a surface term, both extensive. The latter only occurs when
the transverse conductivity is nonzero and owes to conducting surface states.
Simulations on a model Hamiltonian validate our theory.
