The leaves are trigonal; they are very short, and they are laid over each other like scales in four rows.
Trigonal trapezohedron (fig. 74), bounded by six trapezoidal faces.
All other forms, in which the faces are neither parallel nor perpendicular to the triad axis, are trigonal pyramids.
Fig. 73 represents a crystal of tourmaline with the trigonal prism , hexagonal prism , and ...