1) Having threefold symmetry 2) Of a triangle 3) Rhombohedral 4) Symmetric 5) Symmetrical 1) Rhombohedral 2) Symmetric 3) Symmetrical 
one of the crystal systems that is considered by some gem references to be a subsystem of the hexagonal system. A trigonal prism has the effect of being a three sided prism. See Crystal Systems & Forms.
having three angles
• (a.) Having three angles, or corners; triangular; as, a trigonal stem, one having tree prominent longitudinal angles.
<botany> Triangular in cross-section as applied to stems. ... (09 Oct 1997) ... 
Type: Term Pronunciation: trig′ō-năl Definitions: 1. Triangular; relating to a trigonum.
trigonal Triangular; related to a trigonum.
(see crystal systems) Crystals forming under this system have a three fold axis of symmetry 3 horizontal reference axis, equal in length and at 120 degrees to each other.
A crystal system is described by 3 basis vectors. In crystallography, the trigonal crystal system is one of the 7 lattice point groups. Quartz is represented as D3 in the Schoenflies notation of Point groups in 3 dimensions.