Naclstack is a quadrupedal Pokémon whose body is made of rock salt. Its cubic head is white with a few cracks and two eyes in the shape of right angles. Each of the eyes are orange in color with yellow squares at their centers, and they can glow an orange-red. The head is connected to Naclstack's body; the upper half of the body consists of a brown cuboid stone at the front and a rugged, dark brown rock dotted with salt at the back, while the lower body is a single white cuboid. Naclstack's body doesn't connect to its legs and floats in place relative to them. The legs themselves are cube-shaped, with light brown on top and white at the bottom. Each of the cubes have a small brown stone on one of their corners, which seem to act as claws for the Pokémon.
Naclstack attacks by pressing the rock salt inside its body into hardened pellets, which it then shoots out with enough force to puncture an iron sheet. Naclstack also uses its salt to dry out and cure its prey. Naclstack leaves behind bits of salt on its footprints as it walks; these salt grains are gathered by people and used as seasoning for sandwiches.
Naclstack and its evolved form, Garganacl, are the only known Pokémon capable of learning the move Salt Cure. Naclstack and its evolutionary relatives are also the only known Pokémon that can have Purifying Salt as an Ability.
Naclstack is based on rock salt crystals and sedimentary layers of bedrock. Its shape resembles a quadrupedal animal. It may be loosely inspired by ancient Iberian sculptures like the Bicha of Balazote or the Sphinx of Agost, or the verraco granite sculptures. Naclstack's front body composition appears to be inspired by voxel artwork. Its glowing eyes may be based on Himalayan salt lamps.
Naclstack may be a combination of NaCl (chemical formula for sodium chloride; table salt) and stack, possibly in reference to dry stack (method of stone structure building that doesn't use mortar).
Jiodumu may be a combination of 塩 shio (salt), geo- (prefix for earth) or geode, and 積む tsumu (to stack).