Schiedea adamantis is a gynodioecious (having some individuals bearing female flowers only and others bearing hermaphrodite flowers only) shrub species that grows to a height of 3 to 8 decimeters. Its stems are often swollen at the nodes and can be terete, 4-angled, or -ridged, with an erect or strongly ascending growth habit. These stems are glabrous, with internodes measuring 0.5 to 4.1 cm in length, colored green to greenish-purple. The lateral branches of the primary stems typically elongate several nodes below the inflorescence. A fibrous root system supports the plant. The leaves are simple and opposite, with elliptic to elliptic-oblanceolate or oblanceolate blades. The blades are 1.5 to 6 cm long and 0.4 to 2.2 cm wide, with an acute to acuminate apex. The leaf bases are often connate around the stem and cuneate. The surfaces are yellowish-green, and the blades are weakly coriaceous. The margins are entire, slightly thickened, and weakly revolute. The midvein is the only vein evident. The petioles are 0.3 to 0.8 cm long, and stipules are absent. The flowers form in terminal inflorescences that are laterally contracted and somewhat congested. The hermaphroditic inflorescences are 3.1 to 12.3 cm long with 22 to 170 flowers, less congested and broader than the pistillate inflorescences, which are 2.3 to 11.4 cm long with 30 to 140 flowers. The flowers are either bisexual (perfect) or pistillate (4–)5-merous. The calyx of hermaphroditic flowers has sepals 3.4 to 4.5 mm long, while the pistillate flowers have sepals 2.6 to 3.2 mm long. The corolla (petals) is absent. Hermaphroditic flowers have 10 stamens with slightly dimorphic filaments, while pistillate flowers have vestigial stamens. The ovary is superior, with 3 to 5 styles in pistillate flowers and 3 to 4 in hermaphroditic flowers. The fruit is a narrowly ovoid capsule, 3.5 to 5 mm long, with relatively few reniform, compressed seeds that are 0.8 to 0.9 mm long. The seed surface is papillose, with larger papillae on the margins and a starchy perisperm. The true endosperm is absent. It is exclusively found on steep, dry slopes in largely alien shrubland at approximately 125 meters.