The total height of a brilliant is measured between table and culet. In this the work must be carefully done, as the culet of the stone, because of a certain brittleness, is very sensitive and easily damaged. The total height in millimeters serves at the same time as an identifying factor and can be calculated in percentage of the average girdle diameter.
total height x 100
girdle diameter
The total height in percentage of the girdle diameter
does not in any way allow any conclusion to be drawn as to the quality
of the proportions, for the ratio of crown : pavilion and the thickness
of the girdle are much more decisive, for the total height is found from
the addition of these three. Checking the correct relationship of these
proportions is far more important than measuring the total height alone.
Thus, for example, three brilliants which all have a total height of 60
%, could have completely different crown and pavilion heights. One of
them may be cut in the ideal crown: pavilion ratio, another stone
possibly has too shallow a crown and too deep a pavilion, and finally
the third brilliant may also show a total height of 60 % but have a high
crown and shallow pavilion.
The percentages for total height quoted in books and tables are often
given without reference to the thickness of the girdle. Thus, for
example, for the Practical Fine Cut one reads a value of57.6%, which
results from a 14.4 % crown and a 43.2 % pavilion depth. For an ideal
girdle, which is visible to the unaided eye as a thin, light line, 3 to
4 % of the girdle diameter must be added for smaller stones, and 1 to 2
% for larger stones, to obtain the total value.
The height of the crown and pavilion is measured by the angles made by
crown and pavilion facets with the plane of the girdle; the larger they
are, the higher. are 'the crown and pavilion of a brilliant. As these
angle sizes are of extreme importance to the refraction, total
reflection and dispersion of the incident light falling upon a
brilliant, greater attention should be paid, in estimating the total
height, to the size relationships of the crown and pavilion and the
thickness of the girdle. It is not sufficient to measure only the total
height.
| Table comparison method | Crown height |