The White-bellied Mountain-Gem is distinguished by its white stripe behind the eye, white underparts, and bronzy tail. It frequents the canopy and edge of cool, wet forests, and also ventures down to shrub level in gaps and open parts of the forest. Males are notably aggressive and frequently outcompete larger species at flowers. They are especially fond of flowers on large epiphytes and defend them adamantly. During the breeding season, males give an incessant, squeaky jumble of notes as they chase after other hummingbirds.