The genus name was derived from the Greek words rhombos, meaning lozenge and phyllon, meaning leaf, in reference to the somewhat rhomboidally shaped leaves. The plants flower in spring and early summer. The flowers open in the afternoon or evening. The rather strange and outlandish appearance of the species of Rhombophyllum, make them some of the more desirable mesembs to grow.

Rhombophyllum nelii is a compact succulent shrublet similar to Rhombophyllum dolabriforme that branches profusely near the base, resulting in a more open and straggly shrub, the inner stems blackish distally. The leaves are apically divided to various degrees, greyish-green, rough with raised dots, and up to 20 mm long.