The thimble cactus, Mammillaria vetula subsp. gracilis (Mammillaria gracilis), is a small cactus with somewhat slender, cylindrical, bright green body clustering and branching freely, but the offsets easily detached. The offsets are easily knocked off so the plant must be handled with care; however, these offsets quickly root if placed on the ground. The tubercles are roundish, each being tipped by stiff white spines, radiating like a star, making the plant look lace-covered. There are one or more longer dark-brown central spines. Creamy white, bell-shaped flowers appear in spring or in late summer-autumn. Eventually, this cactus forms a large, rounded clumps. Typically subsp. gracilis has only 11-16 radials, often lacks central spines, and flowers are less than 12 mm long.