Shrubs or trees, to 3(–7) m; trunk well defined, to 0.3 m diam.; bark fissured vertically; branches many, spreading or straggling, knotty; crown diffuse. Leaves opposite or fascicled, 1–3 cm, folded at night and often also in heat of day; stipules persistent, subulate, 1 mm, apex acute, somewhat spinescent, glabrous; petiole absent or nearly so; leaflets 8–16, dark green, linear-oblong to linear-spatulate, 5–15 × 2–3 mm, apex obtuse, coriaceous, surfaces reticulate. Pedicels hairy. Flowers axillary, mostly solitary, sometimes clustered, 1.2–2 cm diam.; sepals (4–)5, obovate, to 5 mm, outer smaller; petals (4–)5, usually blue to purple, rarely white, obovate to elliptic, 6–10 × 2–3 mm, base short-clawed, apex often notched; stamens (8–)10, shorter than petals; filaments each with small crenate scale at base; ovary obcordiform, flattened, 2(–4)-lobed, 2(–4)-locular, hairy; style subulate. Capsules becoming orange, obcordiform, flattened, 10–20 mm diam., 2(–4)-lobed, 2(–4)-locular, ± 2(–4)-winged, apex abruptly attenuate-apiculate, reticulate, hairy. Seeds yellowish brown, ovoid to reniform, 10–11 mm.
Phenology: Flowering Mar–Sep.
Habitat: Shrubby vegetation, limestone soils.
Elevation: 0–1200 m.
Tex., Mexico (Chihuahua, Coahuila, Nuevo León, San Luis Potosí, Tamaulipas).
Guaiacum angustifolium occurs in southern, central, and western Texas. The root bark is used as a source of soap, and root extracts are used in folk medicine to treat various diseases. The stems are used for fence posts, tool handles, and firewood.