Rockia sandwicensis is a small tree with an openly branched structure that can grow up to 12–15 meters tall. The stems have relatively thick branchlets, with distal internodes becoming very short and rarely congested to form aggregate nodes. The youngest growth is brown tomentulose and becomes glabrate over time. The leaves are simple and opposite, although some may be somewhat alternate distally. The blades are oblong to occasionally ovate-elliptic or broadly elliptic, measuring 10–28(–31) cm long and 5–10(–16) cm wide. The leaf apex is obtuse or rarely bluntly acute or rounded, and the base is abruptly contracted, rounded to sub-truncate, often with unequal sides. The surfaces are glabrate, and the blades are thick and chartaceous to usually coriaceous. The margins are entire to sinuate. The primary lateral veins are 9–12 pairs, conspicuous, forking, and anastomosing toward the margins, with tertiary and higher-order venation faint but visible, forming areolae of several orders. The petioles are 2–6 cm long, stout, tomentulose, and becoming glabrate. Stipules are absent. The flowers are long-pedunculated, globose to hemispherical, and axillary cymes measuring 4–10 cm long at anthesis, elongating up to rarely 20 cm long in fruit. They are dense to moderately brown tomentose, with the primary branching thyrsoid to umbelloid and the branched portion of the cyme, usually globose, with irregular branching of the cymules. The flowers are unisexual and weakly fragrant, short-pedicellate to sessile. The calyx of staminate flowers is densely tomentose and brownish, with a tube that is prismatic to cylindrical, occasionally somewhat ampliate upward, 3–5 mm long, with oblong lobes measuring 2–4 mm long and 1–2 mm wide, recurved, tomentose externally, and glabrous within. The calyx of pistillate flowers is similar but soon elongates. The corolla (petals) are absent. Staminate flowers have 10–30 exserted stamens, ranging from once to twice the length of the perianth tube, with orbicular anthers. Pistillate flowers have staminodes usually included, occasionally with sterile anthers visible in the perianth throat, rarely exserted so that the flowers resemble staminate flowers except that the anthers are sterile but dehiscent. The ovary is superior, with a pistillode usually included in staminate flowers but sometimes exserted to as much as half the length of the stamens, with a rudimentary stigma that is recurved, flattened, and somewhat pilose. Pistillate flowers have a glabrous style that is exserted to the length of the perianth lobes, with a flattened, lanceolate, or paddle-shaped stigma covered with elongate hair-like branching processes or fimbriate, usually recurved but sometimes straight and erect. The fruit, known as anthocarps, is prismatic, measuring 3–6.5 cm long and up to 7.5 mm in diameter at the widest part, about 1 cm above the base. The upper one-third to one-half usually tapers to a slender sterile rostrum and is crowned by erect, persistent lobes. The five angles are smooth but very glutinous. Each fruit contains one seed closely enclosed by the thin ovary wall. It is found in dry to mesic forests, usually on slopes or in gulches, between 260-1,040 meters.