|
Glossary 355
Talcose. Partaking of the characters of talc. Tallow-Topped. A stone cut with a flattish convex surface. Tariff. Ten per cent, import duty imposed upon cut
diamonds by the United States Government. Tetragonal. Pertaining to a tetragon; having four angles or sides, as a square, quadrangle, or rhomb. Tetragonal System. A
system of crystallisation in which the lateral axes are equal, being
the diameters of a square, while the vertical is either longer or
shorter than the lateral. Called also Dimetric, Mona-dimetric, or
Pyramidal System. Tiffanyite. A hydrocarbon, causing phosphorescence and
opalescence in some precious stones. Top Crystals. Standard grade of diamonds. Torn End. A three-cornered pyramid from the point of
a wassie. Translucent. Minerals so nearly opaque that objects
are scarcely, if at all, visible through them. Transparent. When the outlines of an object can be
seen through a gem distinctly. Triclinic. The
system in crystallography in which the three crystallographic axes are
unequal, and inclined at angles which are not right angles, so that the
forms are oblique in every direction, and have no plane of symmetry. Twinned. Two or more distinct crystals which have
been formed in conjunction. Uniaxial. Having one direction within the crystal, along which a ray of light can proceed without being bifurcated. Unio. The river mussel; the type-genus of Unionidse, with more than 400 species from all parts of the world. Uralian. Minerals from the Ural Mountains, Siberia. Vitreous. Glassy, as glassy lustre. Wassie. A large cleavage of a crystal split for cutting,
as an octahedron divided into two pieces. Waxy. A distinctive lustre, as of the turquoise. Weathering. The disintegration and decay of minerals under the influence of the weather.
|
||
|
|
||