28                    A PRACTICAL TREATISE ON GEMS. The next class of crystals are the semi-tesseral forms; and first, those with oblique faces, often named tetrahedral, from their relation to the tetrahedron. (1.) This form (fig. 8)
	is bounded by four equilateral triangles, has six equal edges with faces meeting at Ï0° 32', and four trigonal angles. The principal axes join the middle points of each two op­posite edges.—Ex., gray-copper ore, boracite, and helvine. (2.) The trigonal dodecahedrons (fig. 9) are bounded by twelve isosceles triangles, and vary in general form from the tetrahedron to the hexahedron. There are six longer edges corresponding to those of the inscribed tetrahedron, and twelve shorter placed three and three over each of its faces ; and four hexagonal and four trigonal angles.—Ex., gray-copper ore, and bismuth-blende. (3.) The deltoid-dodecahedrons (fig. 10) are bounded by twelve deltoids, and vary in general form from the tetrahedron on the one hand, to the rhombic-dodecahedron on the other. They have twelve longer edges lying in pairs over the edges of the inscribed tetrahedron ; and twelve shorter edges, three and three over each of its faces. The angles are six tetra­gonal (rhombic), four acute trigonal, and four obtuse tri­gonal angles. The principal axes join two and two oppo­site rhombic angles.—Ex., gray-copper ore. (4.) The hex-akistetrahedrons (fig. 11) are bounded by twenty-four