The lava cactus is a unique, leafless, clump-forming species characterized by its cylindrical stems, which typically reach heights of 50–60 cm (20–24 inches) and can form clusters up to 2 meters (6 feet 7 inches) wide. The stems feature 16–22 ribs and are predominantly yellow, with variations of green or brown tones. Each areole can bear up to 40 spines, which can grow as long as 5 cm (2 inches). Initially yellowish, these spines darken as they mature. The flowers of the lava cactus are produced singly and are narrowly funnel-shaped, measuring up to 11 cm (4 3/8 inches) long and 5.5 cm (2 1/8 inches) across. The lower part of the flowers is adorned with numerous spines. They bloom during the day, displaying white to yellowish-white interiors. After flowering, the remnants of the bloom remain attached to the fruit, which is a berry that can be red to brown in color, covered with yellow spines, and filled with many small black seeds.