  Half a century ago, Mott noted that tuning the carrier density of a semimetal
towards zero produces an insulating state in which electrons and holes form
bound pairs. It was later argued that such pairing persists even if a
semiconducting gap opens in the underlying band structure, giving rise to what
has become known as the strong coupling limit of an `excitonic insulator.'
While these `weak' and `strong' coupling extremes were subsequently proposed to
be manifestations of the same excitonic state of electronic matter, the
predicted continuity of such a phase across a band gap opening has not been
realized experimentally in any material. Here we show the quantum limit of
graphite, by way of temperature and angle-resolved magnetoresistance
measurements, to host such an excitonic insulator phase that evolves
continuously between the weak and strong coupling limits. We find that the
maximum transition temperature T_EI of the excitonic phase is coincident with a
band gap opening in the underlying electronic structure at B_0= 46 +/- 1 T,
which is evidenced above T_EI by a thermally broadened inflection point in the
magnetoresistance. The overall asymmetry of the observed phase boundary around
B_0 closely matches theoretical predictions of a magnetic field-tuned excitonic
insulator phase in which the opening of a band gap marks a crossover from
predominantly momentum-space pairing to real-space pairing.
