Both sexes of all subspecies have a medium-length straight black bill, dark auriculars, a whitish stripe behind the eye, and a broad slightly forked tail.

[5] Adult males of the nominate subspecies have a dark green crown and back, a bronze rump, and blackish uppertail coverts.

Males of L. a. nobilis have more bronzy green upperparts than the nominate, a purple rump, and purplish-black uppertail coverts.

Both sexes are much darker overall and the male's gorget is violet to royal blue instead of the red to reddish purple of the other four subspecies.

[5] When compared to similar sister species, it appears the color differences between this and others was a relatively recent evolutionary occurrence.

It tends to feed in the lower and mid-story of the forest by trap-lining, visiting a circuit of nectar sources rather than defending patches of them.

[8] The female builds the nest, a deep cup of plant fibers bound with spider silk and decorated with moss and lichen.

The amethyst-throated mountaingem's song is "a quiet chatter made up of a two-syllable introductory phrase followed by a mixture of notes arranged into very complex vocalizations."