(Hemimorphic-tetartohedral).
Here there is only the triad axis of symmetry, which is uniterminal.
The general form {hkl} is a trigonal pyramid consisting of three faces
at one end of the crystal. All other forms, in which the faces are
neither parallel nor perpendicular to the triad axis, are trigonal
pyramids. All the prisms are trigonal prisms; and perpendicular to
these are two pedions.
[Illustration: FIG. 76.--Crystal of Dioptase.]
The only substance known to crystallize in this class is sodium
periodate (NaIO4.3H2O), the crystals of which are circularly
polarizing.