oward the front of the stone. The well-cut diamond is a very
brilliant object, viewed from the front.

We must now consider how the "fire" or prismatic color play is produced,
for it is even more upon the display of fire than upon its pure white
brilliancy that the beauty of a diamond depends.

CAUSE OF "FIRE." As we saw in Lesson X. (which it would be well to
re-read at this time), white light that changes its course from one
transparent medium to another at any but a right angle to the surface
involved, is not only refracted (as we saw in Lesson II.) but is
dispersed, that is, light of different colors is bent by differing
amounts and thus we have a separation of the various colors. If this
takes place as the ray of light leaves the upper surface of a brilliant
the observer upon whose eye the light falls will see either the red, or
the yellow, or the blue, as the case may be, rather than the white light
which entered the stone. If instead, the dispersion takes place as the
light enters the brilliant the various colored rays thus produced will
be totally reflected back to the observer (slightly weakened by
spreading, as compared to the direct or unreflected spectra). Thus
dispersion produces the "fire" in a brilliant.

Other materials than diamond behave similarly, but usually to a much
smaller extent, for few gem materials have so high a refractive power or
so great a dispersive power as diamond.

Having considered the theory of the brilliant we may now take up a study
of the methods by which the exceedingly hard rough diamond is shaped and
polished.

CLEAVING DIAMONDS. If the rough material is of poor shape, or if it has
conspicuous defects in it which prevent its being made into a single
stone, it is cleaved (_i. e._, split along its grain). Hard as it is,
diamond splits readily in certain definite directions (parallel to any
of the triangular faces of the octahedral crystal). The cleaver has to
know the grain of rough diamonds from the external appearance, even
when the crystals, as found, are complicated modifications of the simple
crystal form. He can thus take advantage of the cleavage to speedily
reduce the rough material in size and shape to suit the necessity of the
case. The cleaving is accomplished by making a nick or groove in the
surface of the rough material at the proper point (the stone being held
by a tenacious wax, in the end of a holder, placed upright in a firm
support). A thin steel knife bla