Twigs are 4-sides, finely hairy and brownish when young. The apex is rounded or blunt-pointed and the base pointed. Upper surfaces are green and the lower surface is pale green, velvety with brownish hairs, with minute, golden, shiny gland-dots. Three or four prominent, ascending, curved veins are present on either side of the mid-vein. Petioles are 1 to 2 cm in length and brownish-hairy.
Flowers are perfect and zygomorphic.
The corolla is bluish or purplish, finely hairy outside with long hairs inside. Palo de nigua was collected for the first time by Sintenis in 1885 while he was collecting specimens for Urban and Krug. It has been reported from Guajataca but this may be the same site as that reported as San Sebastian. In the 1960's Woodbury collected the species at Susua Commonwealth Forest. It has always been observed as only single trees.