In the every day practice of the jeweler and diamond dealer, the question often arises whether the modernizing of old-cut diamonds is sensible and worth it. It is important to advise the customer and to weigh up all the factors, which influence positively and negatively the value of the stone before and after the re-cutting.
1. Should every old-cut diamond be re-cut ?
Some old-cuts have been developed from the shape of the natural octahedron and are characterized by a square base (Fig394). Such cuts were produced up to the middle of the 19th century, and they have today apart from their material value also an antique value. The table of such an old-cut is very small. about 40-48%, the culet octagonal and large. The fifty six facets of the crown and pavilion vary in size. The point of intersection between the main facets and the girdle facets is very near to the girdle, perhaps in the proportion 25 : 75. The total height is much too high in proportion to the girdle diameter, the relation between crown and pavilion is usually 1 : 1 (Fig 393).
The weight loss caused by modernizing these old-cuts is usually quite big because of their square base, so that the re-cutting is often not viable. It would also destroy any antique value.
| Fig 394 Old cut brilliant - up to about 1850 |
 |
 |
|
| Fig 396 Old cut brilliant - from .about 1850 | Fig 393 Proportions of an old cut brilliant |
Brilliants which were cut later than 1850, often have an approximate round diameter (Fig 395). The table is still very small, the culet still quite large, the arrangements of the facets is symmetrical and the main facets of the same size, but the intersection between main and girdle facets in the pavilion cut in a proportion 40: 60. The "younger" old cuts are generally quite suitable for re-cutting, as they do not have an antique value, and the weight loss because of their existing circular shape is much lower.
2. From what size is the modernizing of an old-cut viable?
Before examining this question, it must be established that the modern cut brilliant should be at least as valuable as the old cut stone plus the cutting costs. The minimum size for an old cut brilliant should be 1 .00 ct. Only in the case of first quality stone re-cutting might be advisable for half-caraters.
3. Can color and clarity grades be improved by modernization?
The color of a clumsily proportioned old-cut diamond when re-cut to ideal proportions with maximum yield of light and dispersion can be improved by one to two grades. It might be possible to remove inclusions which are situated in the old cut near to the surface of a high crown, near to a pavilion facet and near to the girdle this would include external features - so that the "new" brilliant also in this way can be graded one to two grades higher and thus be higher evaluated.
4. How high are the re-cutting costs?
The costs for modernizing and re-cutting are roughly $100 per carat of the remaining weight.
5. Is there a risk in re-cutting an old-cut?
Yes, especially where the brilliant shows inclusions in the shape of cleavage or fracture cracks which could expand under pressure and temperature increase during cutting. Even diamonds without these features or with only very small inclusions can - rarely - develop cracks because of inner tension, which can then expand and "blind" the whole stone. The jeweler should point out this risk to his customer, as it is the customer who has to bear any loss or damage.
After careful examination of the above factors, the following questions have to be asked before re-cutting:
- What dimensions and what carat weight can be expected from the brilliant re-cut to modern dimensions?
- How big is the approximate weight loss?
- Can one calculate dimension and weight of the re-cut stone?
Old-Cut Brilliants
Old cuts can be subdivided according to their re-cutting possibilities according to the following system:
- Old-cuts with square-round or approximately round girdle diameter with high crown or deep pavilion. The total height is relatively too high when compared to the girdle diameter (Fig 396).
- Old-cuts with square-round or approximately round girdle diameter with high crown and very flat pavilion. The total height is relatively too flat when compared to the girdle diameter (Fig 397).
The tolerance of the total height of a very well cut diamond lies within 54-62%:
| Minimum value | Maximum value | |
| Height of crown Width of girdle Height of pavilion |
11% 1% 42% |
15% 2% 45% |
| Total height | 54% | 62% |
However, in the case of most commercially "normal" brilliant cuts, the height can be limited to within 58-62 %, i.e. within this zone the total height of a re-cut brilliant with very good proportions should be accommodated.
First, the relationship between the total height of the old-cut to the girdle diameter (R Ø) has to be established. The height is divided by the girdle diameter.
height
R Ø (smallest Value) X 100
If the height is definitely larger than 62 %, the old-cut is too high; if less than 58 %, the old-cut is too flat. In the case of a brilliant which is too high, the total height must be reduced.
 |
 |
| Fig 396 Modernizing high old cuts - the height is reduced | |
 |
 |
| Fig 397 Modernizing flat old cuts - the girdle diameter is reduced | |
A new height must be established:
The diameter of the girdle is measured in various directions; this gives very uneven results, especially in the case of square-round shapes. Then the smallest diameter in millimeters is selected.
This smallest diameter = 100 %.
The maximal allowable total height of 62 % is then calculated to avoid unnecessary loss of weight and to use as much as possible of the existing "diamond material" of the old-cut.
The formula for calculating the new height is:
Height = R Ø (smallest Value) X 0.62
The result shows the total height in millimeters of the new brilliant after re-cutting.
Taking this calculated height and the measured smallest girdle diameter the weight can be estimated according to the following formula:
carat weight = height(new) x R Ø2 X 0.0061
In the case of a square-round diameter, the loss in weight is about 20-30 %, in the case of brilliants with constant diameter about 5-15 %.
In the case of old-cuts where. the total height is definitely too low (under 54 %), the girdle diameter must be decreased in order to achieve ideal proportions between the total height and the girdle diameter.
A new girdle diameter must be established:
The total height is measured in millimeters. The height is taken as = 58 %.
A small percentage value recommended for a "very good" cut is selected so that the girdle does not have to be reduced too much in order to save weight. The girdle diameter of 100 % is calculated according to the following formula:
R Ø = height x 100/58
Taking the measured total height and the calculated girdle diameter the weight can be arrived at using the known formula:
carat weight = height x R Ø2 X 0.0061
In the case of a brilliant with square-round base the loss of weight is about 30-40 %, where the shape is more circular about 10-20 %.
| Instruments for the Approximate Estimation of Weights of Brilliants | Damaged Diamonds |