Hexagonal bipyramid of the first order, bounded by twelve isosceles
triangles (fig. 70; p and u in fig. 80); indices {101'1}, {202'1} ...
(hoh'l). The hexagonal bipyramid so common in quartz is geometrically
similar to this form, but it really is a combination of two
rhombohedra, a direct and an inverse, the faces of which differ in
surface characters and often also in size.
Hexagonal bipyramid of the second order, bounded by twelve faces (s in
figs. 79 and 80); indices {112'1}, {112'2} ... {h.h.2'h'.l}.
Dihexagonal prism, consisting of twelve faces parallel to the hexad
axis and inclined to the vertical planes of symmetry; indices {hiko}.
Hexagonal prism of the first order {1010}, consisting of six faces
parallel to the hexad axis and perpendicular to one set of three
vertical planes of symmetry (m in figs. 71, 78-80).
Hexagonal prism of the second order {112'0}, consisting of six faces
also parallel to the hexad axis, but perpendicular to the other set of
three vertical planes of symmetry (a in fig. 78).
Basal pinacoid {0001}, consisting of a pair of parallel planes
perpendicular to the hexad axis (c in figs. 71, 78-80).
Beryl (emerald), connellite, zinc, magnesium and beryllium crystallize
in this class.