White-throated Mountain-gem is a medium sized hummingbird with a long, straight, all black bill. The male is mostly dark green, with a white throat and postocular stream; the female shares the white postocular, but is entirely cinnamon rufous on the underparts. White-throated Mountain-gem has a restricted geographic distribution, in the highlands of southern Costa Rica and western Panama, but this small range encompasses two quite distinct subspecies: cinereicauda, of Costa Rica, in which the male has a blue forecrown and a gray tail; and castaneoventris of Panama, in which the forecrown of the male is bluish green, and the tail is green with a bluish black subterminal band. These two taxa are recognized as separate species by some authorities. Throughout its range, this species occupies humid montane forests, especially oak forests. Although this species places the nest in the understory, it frequently forages in the canopy. White-throated Mountain-gems are territorial at patches of flowers, and are dominant to smaller species, but in turn is subordinate to the larger Fiery-throated Hummingbird (Panterpe insignis). Male White-throated Mountain-gems often prefer epiphytic flowers in the forest interior while females are more likely to visit shrubs in second growth or in coffee plantations.