[5] These three species form a closely related group that evolved some 3.5 million years ago and has diversified since.

[7] Both sexes of all subspecies have a medium-length straight black bill, a white to pale buff stripe behind the eye, and a fairly long tail.

The rest of their upperparts are metallic bronze-green that shades to bluish or grass green on the uppertail coverts.

[5] Adult females of the nominate subspecies have bright metallic green upperparts that are somewhat bluish on the uppertail coverts and a bit bronzish elsewhere.

The lower half of the outer tail feathers are mostly black with pale gray tips.

[5] Immature males of the nominate subspecies are much like the adult female, but with a dusky chin and throat and brownish gray undertail coverts with dull white margins.

[5] Males of subspecies L. c. homogenes are similar to those of the nominate but have a darker gray breast and belly.

[5] Subspecies L. c. pectoralis of purple-throated mountaingem is found from far southwestern Nicaragua to the Cordillera de Guanacaste in northwestern Costa Rica.

[3][5] The species inhabits humid montane evergreen forest and cloudforest, where it favors steep slopes and broken terrain.

The purple-throated mountaingem's song is "high, thin, and dry, a complex medley of sputtering and warbling notes."

A frequently heard call is variously described as "trrrt" or "a sharp, penetrating, buzzy zeet or zeep."