rhomboedral

Rhomboedral, often spelled rhombohedral in modern usage, is an adjective used in geometry and crystallography to describe a shape or lattice based on a rhombohedron. A rhombohedron is a six-faced polyhedron whose faces are congruent rhombi; all edges are of equal length, and opposite faces are parallel. It is a type of parallelepiped and can be formed by skewing a cube along a body diagonal. The angles between edges are not right angles, and the dihedral angles between adjacent faces are generally oblique.

In geometry, the rhombohedron is a common example of a solid with equal edge lengths but non-orthogonal faces. Its faces are parallelograms, specifically rhombi, and the figure is determined by three edge directions that are equally long but not mutually perpendicular. This makes the shape a useful reference in discussions of skewed three-dimensional forms and crystal-like geometries.

In crystallography, rhombohedral refers to crystals whose lattice or unit cell can be described as a rhombohedron. The term is historically tied to the trigonal crystal system and to a rhombohedral lattice that can be described in a rhombohedral setting or in a hexagonal setting with appropriate parameters. Minerals such as calcite and dolomite exhibit rhombohedral lattice characteristics and may show rhombohedral cleavage or habit. The concept emphasizes symmetry and the geometric relationship between lattice vectors rather than a strict cubic or orthogonal framework.