124 The Topaz.
of a very large size without defects, and many of the large stones
commonly sold as such are merely rock-crystal or quartz of a yellow
colour, such as cairngorm and others, between which and the true
topaz, however, there are many important differences, as a reference
to the chapter on quartz will show.
topaz belongs to the trimetric system of crystallization. Its cleavage
is basal ; that is, it breaks with a flat surface, at right angles,
with or across the prism. Its sperine gravity is 3·5 to 3.6; it is numbered eight in the scale of hardness, scratches quartz easily, and possesses double refraction in a slight degree.
Its chemical composition is—