a particular form is known as a polymorph. Tetrahedron (plural: tetrahedra) is a polyhedron composed of four triangular faces, three of which meet at each vertex . References 1. NIOSH-Department of Labor Fact Sheets. Preventing silicosis. October, 1996. 2. Parkes, RW Occupational Lung Disease.3d edn. pp.285-323, Butterworth-Heinneman, Oxford , UK ,