62 PRECIOUS STONES
Though
a buyer of diamonds cannot always tell by measurement whether or not a
stone is cut to its best proportions, he can decide the question by its
appearance. A stone properly proportioned shows an equal distribution
of light and brilliancy. The centre under the table is as full of
light, reflected by the back facets, as the edge of the stone from the
table to the girdle, where it is cut up to receive light and reflect it
from the surface. If the stone be too shallow or too deep, the light
entering the stone is not thrown sufficiently into its body, but passes
out around the culet, leaving a dead centre. In stones cut too deep
this results in a black " well" through the centre of the stone; if too
shallow, there is a different but similar effect of weakness which
produces a glassy appearance. Such stones are termed " fish-eyes."
Since
the trade have found how important it is to have a proper " spread" to
the stone, and that one of the same diameter weighing less may be more
brilliant than another which is heavier, there has been a tendency to
demand stones too shallow for the best results. It is important that
there shall be sufficient depth from the girdle to the culet; nor does
this sufficiency, coming on the under side, as it does, almost to a
point, add much to the weight. Thickness should be avoided at and above
the girdle. If a diamond shows a full volume of brilliancy both when
held near by and at arm's length from the eye, its proportions cannot
be far wrong. If it shows weakness or a dead centre, it is not a good
stone to buy or sell, though there is a demand for them, generally from
people who have a mistaken notion that thin stones give them more show
for their money. The actual size of a " fishy" stone never impresses
the eye so much as the brilliancy of a smaller well-cut stone. Dealers
should remember that while in " lumpy" stones they pay for weight of
material which is detrimental, a sufficient depth is necessary to
secure that brilliancy which is the chief beauty of the diamond.
Figures on Plate XIX. illustrate the manner in which a ray of light
trav-
