Mountain-gems (Lampornis) are medium-sized hummingbirds with dark, straight bills, fairly long tails, dark auriculars, and a whitish post-ocular stripe. The Purple-throated Mountain-gem is sexually dimorphic. The males are primarily green, with a purple gorget. The upperparts of the female are green, but the underparts are pale rufous, and the female lacks a gorget.
Does not overlap with any other particularly similar species.
The following description is based on Ridgway (1911):
Adult male: Forecrown and crown glittering emerald green, varying to bluish green. Remaining upperparts metallic bronze-green, becoming more bluish or grass green on uppertail coverts. Post-ocular streak white. Auriculars and suborbital area dusky bronze-green. Rectrices dull blue-black. Remiges dusky purplish brown; inner secondaries mostly bronze-greenish, especially on outer webs. Primary coverts dull black. Chin and throt metallic violet or purple. Breast bright metallic green, flanks and belly duller, more bronzy green or gray. Undertail coverts deep bronzy gray margined with paler gray.
Adult female: Upperparts bright metallic green, usually more bluish on the uppertail coverts and more bronzy proximally. Lores, subocular region and auricular region blackish, auriculars bordered above by a whitish or pale buff post-ocular stripe. Central rectrices duller metallic green or bronze-green; basal half of remaining rectrices similar, but blackish subterminally, the outer two or three pairs broadly tipped pale gray. Remiges dusky, faintly glossed with violaceous. Throat, breast, and belly tawny-ochraceous; undertail coverts dull white to pale tawny buff.
Immature male: Upperparts (including tail) as adult female. Chin and throat dusky, center of throat with a few tawny-ochraceous feather. Dusky of throat shades into dull metallic bronze-green on lower throat and breast; belly tawny-ochraceous. Undertail coverts brownish gray, broadly margined with dull white.
The Purple-throated Mountain-gem has a straight bill of medium length (male: ca. 21.4 mm, female: ca. 22.1 mm; Feinsinger 1974, Snow 1977). This mountain-gem is a relatively large hummingbird: males weigh approximately 5.6 g, and females 4.2 g (Feinsinger 1974, Snow 1977). Wing length of males (ca. 63 mm) is slightly longer than that of females (ca. 56 mm) (Feinsinger 1974, Snow 1977).