culet
1-6th to 1-5th of the table ; but according to some modern experts,
both these facets, but chiefly the former, may be reduced with
advantage below these proportions. Two of the most famous diamonds of
the world show large departures from the typical proportions of a
brilliant : The Koh-i-nûr in its present form is far too broad for its
depth or thickness ; the Regent is a good deal too thick for its
breadth. But the same rule of proportion, al though it may hold good
for such diamonds as admit of being subjected to it without extravagant
loss of weight, must be modified with stones of other species, and
especially with coloured stones.- With colourless topazes, sapphires,
etc., the surfaces and inclinations of the facets must be modified to
suit the refactive indices and other optical constants of these
minerals ; with coloured stones, if pale (certain alexandrites for
example), greater depth must be secured ; if dark in hue, then greater
" spread " and less depth (deep red garnets furnish instances).
The
style of cutting known as the step-cut or trap-cut is adopted for the
emerald and many coloured stones. It is subject to rules of proportion
far less strict than those devised for the cutting of the diamond in
the brilliant form. Each species of stone needs special study, that the
typical step-cut may be so modified as to bring out the full beauty of
the gem. The fault most common with step-cut stones is the too great
breadth of the table, for the internal reflections from the lower
facets are best seen through the sloping bezels of the crown, not
through the flat surface of the table. In the step-cut (fig. 14) we
have then a table, two or more sloping step facets, and then the
girdle, while the lower part of the stone (fig. 15) is cut into three
or more sets or zones of
