Like the similar grusonia grahamii
, which inhabits the same region of far west Texas (in addition to other areas), grusonia aggeria
can be distinguished from most other members of this small genus by having spines that are cylindrical, tapering evenly to a sharp point, rather than being partially flattened. Spines are colored brown, black or grey, and usually number between 5 and 15 per areole, growing mostly towards the top of the stem. Spine lengths range from 1 to 2 inches. Stems are evenly cylindrical or thicker towards the tip (clavate), and about 4 inches long. Bright yellow flowers are followed by pale yellow fruits, which become grey as they age.