What is a rhombohedron in mineralogy ?

Rhombohedron : definition

Theoretically, a rhombohedron is a closed parallelepipedal polyhedron composed of 6 similar diamond-shaped faces. In fact, in crystals, if the number of faces is constant, these do not always meet the geometric definition of a rhombus (quadrilateral with four sides of the same length). However, they keep parallel sides two by two.