THE COMMON DRAWBACKS IN

CUTTING OF BRILLIANT ROUND DIAMONDS

AND THEIR CORRECTION

 

By Joe C. C. Yuan -- G.G., D.G.A.

 

The primary reason that people like wearing diamonds is that diamonds have the best brilliance, dispersion and scintillation among all precious stones.

Every diamond glitters under the jeweler's lamp irrespective of the cutter's workmanship. If we put the diamonds under close scrutiny, however, we find out that each diamond has its unique gleam. The author of this paper attempts to enumerate the drawbacks of brilliant round diamonds caused by poor cutting and suggest methods of improvement.

 

  1. Bottom Angle Too Deep

    2. Diamonds have a refraction rate of 2.417, which is very high among all precious stones. When the light goes into the diamond via its crown and is all reflected, the most beautiful gleam will be transmitted. For this to happen, the angle between the pavilion main facet and the girdle is of utmost importance. If the bottom angle is too steep, say over 43o, the center portion of the table will be too dark when viewed from the crown (See Fig. 6-1). For commonly cut diamonds, the bottom angle should preferably be within 42.5o. When the bottom angle is too steep, it can be recut. For every degree reduced, about 1.2% of the weight will be lost.

     

  2. The bottom angle is too shallow

    3. Thin diamonds have fish eye effect (See Fig. 6-2). The white circle on the table is the reflection of the whitish girdle. The shallower the bottom angle, the smaller the white circle. When the table is not small enough to cover the white circle, or the white circle can be seen on the table when the diamond is tilted from side to side, all this will count as fish eye effect.

    The depth of the bottom angle must match the size of the table. Shallower angles goes with smaller tables while steeper angles go with larger tables. One of the best examples is shown in Fig. 6-3, where the circle from the reflection of the girdle on the table is slightly larger than the table. Therefore, the reflection of the girdle is visible only when the diamond is tilted over 15o. This is among the best cases. Bottom angles that are too steep or table are too big will cause less than desirable effect.

     

    The proportion of the bottom angle to the percentage of the table is the best when they are matched as follows:

    Bottom Angle

    403/4 o

    41o

    411/2 o

    42o

    421/2 o

    Table

    53%

    54%

    57%

    60%

    63%

    P>The fish eye effect can be eliminated by reducing the size of the table. The loss of weight is about 1-5%, depending upon the depth of the original crown angle. Another way of improvement is to make the bottom angle steeper. This method requires reducing the diameter of the girdle or the thickness of the girdle, thus causing a 2-5% loss of weight. To avoid loss of weight, the rough, opaque girdle can be polished into transparent facets, resulting in 0-0.1% loss of weight. By doing so, the white circle of the fish eye becomes transparent and much less noticeable.

     

  3. Crown angle is too steep

    4. The angle of the kite facet of the crown to the girdle is called the crown angle. Upper limit in GIA standard is 35o, while in actuality it is 35.9o. For all diamonds with a crown angle over 36o, GIA will add the comment "THE CROWN ANGLES ARE GREATER THAN 35o" on the certificate.

    When the crown angle is greater than 36.5o, the table is larger than 62%, and the bottom angle is from normal to slightly steep, a milky reflection will be seen inside the table (See Fig. 6-4). Methods of improvement are: (1) Reduce the crown angle to less than 36 o. (2) Reduce the size of the table to less than 60%. This will eliminate the milkiness.

     

  4. The crown angle is too shallow

    5. When the crown angle is less than 30o, GIA will add the comment "THE CROWN ANGLES ARE LESS THAN 30o" on the certificate. In fact, a crown angle of 30o is the critical angle that separate good crown reflection and poor crown reflection. When a diamond has a crown facet less than 30o, the reflection will appear lack in luster (See Fig. 6-5). One way to improve it is to make the crown angle steeper. Likewise, if the girdle is not thick enough, the girdle should be reduced accordingly. In addition, the size of the table should match the depth of the bottom angle.

     

  5. The table is either too large or too small

    6. The size of the table is in adverse proportion to the crown facets. Larger the table, smaller the crown facets. Likewise, smaller the table, larger the crown facets. The dispersion of the diamond is derived primarily from the crown facets. The brilliance is caused by the light that goes in from the crown and then is reflected by the bottom facet. So the size of the table must be within a proper range. From actual experience, the table size range between 53% and 63%, when well matched by the bottom angle, produces better to the best results.

    To reduce the size of table, if by lower the crown angle, every 1% of reduction in table size will result in 0.5% loss of the diamond weight. Another way is by making "double star" as shown in Fig. 6-6, which table size can be reduced by 4-6% while weight loss about 1%.

     

  6. The star facet is too short

    7. The standard proportion of the star facet to the upper girdle facet is 5:5. In some diamonds, especially those made by old-fashioned cutters, have small star facets that are in 4:6 or 3:7 proportion to the upper girdle facets. The result of this is an elongated kite facet (Fig. 6-7). Short star facets also cause poor refraction.

    "Arrow and Heart" cut, prefer longer stars approximately 6:4 or 7:3 to the upper girdle facets.

     

  7. The height of the lower girdle facets

    8. The lower girdle facet should preferable account for 85%-90% of the area from the girdle to the culet (See Fg. 6-8). It will present a decent view from the top when the height of the lower girdle facets take up 80% or less. The eight pavilion main facets, when viewed from the top appear coarse and broad (Fig. 6-9), will result in poor refraction and lacking in luster. To increase the height of the lower girdle facets will cause approximately 0.3-1% of loss in weight.

     

  8. The arc of the lower girdle facets

    9. When the lower girdle facets are cut, the mountain-shaped arc formed by two neighboring lower girdle facets at the girdle must be even (Fig. 6-10). When viewed from the table, a "lotus" shape can be seen (Fig. 6-11). The eight triangles evenly form a circle, thus producing the best reflection. If the mountain-shaped arcs are too high, as shown by Fig. 6-12, which are due to not enough digging of the lower girdle facets. When viewed from the top, as shown in Fig. 6-13, every triangle of the "lotus" is short and broken and the brilliance is poor. To correct this will require a 0.5-1.5% loss of weight. However, when the mountain-shaped arcs are too low as shown in Fig. 6-14, due to the over digging of the lower girdle facets; when viewed from the top, as shown by Fig. 6-15, the "lotus" is formed by elongated triangles. The brilliance are not much affected, though.

     

  9. The natural at the lower girdle
    10.

    Naturals are mostly coarse and uneven (Fig. 6-16). Most people may think the diamond are broken. If the area isn't too large, additional digging on the lower girdle facets will eliminate or reduce it. The remaining natural can be polished into an extra facet (Fig. 6-17).

     

  10. The girdle is too thick or too thin
    11.

    A girdle that is too thick is not pleasing to the eye. It also makes the diameter of a diamond of the of the same weight appear smaller. To make the girdle thinner, additional digging has to be done on the lower and upper girdle facets, thus making most of the girdle thinner or at least not noticeably thick. The loss of weight is about 0.5-2%. If the girdle is too thick, Solstar 168 cut can be used (Fig. 6-18), with a 1% loss in weight. Another way is to cut a level of facets at a much steeper angle (Fig. 6-19) than that of the lower girdle facets. This will reduce most of the thickness of girdle with 2-3% loss in weight.

    When the girdle of the diamond is too thin, it is easily broken in the setting process, and during wearing (Fig. 6-20). To make it thicker will cause about 0.1-0.2% loss of weight. If the girdle is already broken off a small piece, the repairing will cause about 0.2-3% of loss in weight.

     

  11. Bearding girdle

    12. When rough diamonds are processed, forceful bruting often result in thin vertical white lines on the girdle (Fig. 6-21). The lines, if not too deep, can be polished off by faceting the girdle, causing 0.1-0.3% loss in weight.

     

  12. Chipped culet
    13.

    There are three kinds of culet: 1) NONE; 2) Transparent small facet; 3) Break or chips (Fig. 6-22). The ideal one is with a very small transparent facet. The reason for this is that the culet is protected and not easily broken. Some of the jewelers and consumers prefer the pointed culet is an incorrect idea. If the culet has obvious damage or chips, the repair will cause 0.5-2% of loss in weight, depending on the severity of the chips. To repair the culet, Solstar 66 cut can be used, with a 0.1% loss in weight, while Solstar 128 cut causes 0.5% loss in weight.

     

  13. Damage to the surface

    14. Diamond is a hard substance, but it doesn't have good toughness. When it is hit, it can be broken on the cleavage plane. In the process of setting or wearing, accidental collision will cause little chips or little nicks (Fig. 6-23 and 6-24).

     

  14. Obvious polishing lines

    15. When polishing is not well done, many parallel transparent lines are left on the surface of the diamond, similar to the lines on our fingernails. This results the refraction of diamond to be not sharp enough. If the refraction is too poor, polishing the entire diamond will cause about 1% of loss in weight.

     

  15. Cloud and Poor of brilliance
    16.

    If the diamond appears clouded over or lacking in brilliance, there are three causes for it: 1) External burns, as shown by Fig. 6-25; 2) Internal cloud-like content, as shown by Fig. 6-26; 3) Fluorescent. External burns on the surface may be caused by the excessive heat when the diamond is cut and polished; or they may be caused by inadequate protection when the setter is soldering the diamond ring. Mishandling in the plating process will cause transparent or off-white scars on part of or on the entire surface of the diamond. These external problems can be fixed by polishing of the surface and the brilliance will be restored. The loss of weight ranges from 0% to 2%, depending on the size or severity of the damage.

    Internal clouds or extremely strong fluorescence also affects the diamond's brilliance. To improve the refraction of the diamond, re-designing, re-cutting and re-polishing will be needed in terms of proportion, angles and polishing. If a white plating finish is added to the metal which the diamond is set on, the brilliance of the diamond can be enhanced. This process will cause about 2-7% of loss in weight, depending on the make of the original diamond.

 

Conclusion

On the international market, good make diamonds account for less than 5% (diamonds over 0.25 carat). Most of the diamonds have imperfections caused either by conservation for the weight or by the cutters' limitations in knowledge and skill. In principle, a diamond is good enough if there is no imperfection to the naked eye. If the refraction is poor or has drawbacks to the naked eye, then some repair should be taken into consideration. The improvement will make the diamond much more appealing to the customers.