A skeletal crystal is one that develops under conditions of rapid growth and high degree of supersaturation. Atoms are added more rapidly to the edges and corners of a growing crystal than to the centers of crystal faces, resulting in either branched, tree-like forms or hollow, stepped depressions. Branched crystals are considered to have a "dendritic" habit; the hollow stepped crystals are referred to as "hoppers."
While the concentration gradient of matter to a growing crystal tends to be highest at corners and edges of faces, at low levels of supersaturation the overall growth rate is uninfluenced by local fluctuations in the degree of supersaturation, since flat, polyhedral faces are the general rule among crystals. Only above a critical level of supersaturation will the high corner and edge concentration gradients promote most rapid growth at these positions with the consequent development of skeletal forms.