- The loupe
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The loupe has always been one of the tools of the gemologist arid diamond merchant and is indispensable in the examination of inclusions. For clarity grading of diamonds the 10 x magnifying loupe is today the international standard tool.
The optics of the loupe should be of high quality; it should be achromatic and aplanatic.
Aplanatic means that any error caused by the widening
of the view field are corrected. The so-called "spherical
aberration" causes
deviation from the true image. in the case of. very curved lenses the focus
of rays at the edges lies nearer to the lens than the focus of the rays
nearer to its axis. The focus is then not clear but forms a cloudy circle.
In order to avoid this, lenses of various curvature are combined, or the
rays at the edges are screened off. permitting only the parallel rays near
the axis of the lens to pass through.
The achromatic loupe consists of two lenses of unequal
power and opposite curvature, collecting light of all wavelengths at one
focal point and thus avoided achromatic aberration. For this bi-convex lens
crown glass is used and combined with the less dispersive (bi-concave) lens
of flint glass. Because of the two different dispersions of the two types
of glass it is possible to refract two different colors, i.e. short-wave
violet blue and long-wave red orange onto a common focus.
Today on the market there are many good gem loupes which differ, in the size of their field of vision. A large field of vision is particularly advantageous (Fig 151 )
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The large field of vision of 18 mm and precise
correction yield an image of great color fidelity (achromatic)
and freedom from distortion (aplanatic), with unusual sharpness
and luminous intensity. This loupe can also be provided with 18-fold
and 25-fold magnification. Fig151 A loupe of the Firm Schneider |
As lens glass is very soft, it is advisable to close the lens after use to protect it from damage.
The advancing development of science and technology has brought improvements and innovations in many subjects, including gemology.
As in the case of color-grading with standardized light source or with the help of a color measuring apparatus, the gemologist and diamond specialist. when grading for clarity. can call on the help of instruments which because of their technology guarantee greater reliability. In the practice of diamond grading experts use the diamond microscope for the grading of clarity as this allows reliable and comfortable working especially if grading is to be carried out over a few hours. The microscope is also essential in the production of true-to-nature identity drawings which are today an important part of the expertise. Especially in doubtful cases is the microscope used for testing the clarity, although in borderline cases the 10 x hand loupe is the deciding factor. But today there is no reliable diamond testing laboratory which does not use the microscope when compiling a grading expertise.
The gem microscope is fitted with a binocular lens system and thus enables a calm observation of the object and rendering a plastic image. The lighting is constant, the light exact and diffuse, the stone is held in a movable grip which is fixed to the instrument.
Modern instruments are often reproached with the idea that they tempt the grader away from the standard 10 x magnification. and induce him to test stones "scientifically" with a higher magnification resulting in a stricter grading. This reproach, however, does not apply because no jeweler or gem dealer will degrade the quality of his stones. The reason a microscope is used instead of a hand loupe is not to increase the already very high demands which in any case are very exacting, it should merely give greater reliability to the expert gem tester when grading diamonds.
Apart from the optical advantages of such modern instruments, they also are of benefit to the jeweler in the direct contact with his customers. Up to now it has not been possible for the jeweler to demonstrate to his customer the, inclusions in the various clarity grades of a diamond, as it is only in rare case that a buyer has enough experience to recognize inclusions with the help of a hand loupe.
However, with the help of a microscope with the stone' firmly fixed, and with the jewelers description of the inclusion. even' a layman will be able to see internal and external features of the diamond and will thus obtain a picture of the varying quality groups. Convinced of the objectivity of the instrument, as well as the expert knowledge and the fair advice of the jeweler, the customer will gain confidence and will be more inclined to buy.
Frequently there is a demand for loupe clean diamonds, partly because this term is well-known from press, conversation and advertisements. Diamonds which do not reach this level are thought of by the layman as inferior. If however, the buyer has the opportunity to view diamonds of the VVS or VS grade with a 10 x magnification, he is astounded by the insignificance of the inclusions and the decision to buy is taken, especially as the price is that much lower than that of a diamond of the same size and color which is loupe clean. It can even happen that diamonds of the clarity grade SI or P1 fascinate the buyer by the inclusions which give the stone character and individuality, that he will value a brilliant because of its special inclusion picture more than an ordinary loupe clean stone. An additional inducement is that the buyer can purchase for the same prise a substantially larger 81 stone than a loupe clean stone because of the price structure. Also the customer who wants a loupe clean diamond can convince himself of the clarity of a stone, making a second opinion by another - maybe prejudiced - jeweler unnecessary.
It must now be underlined, that it is wrong to designate inclusions as faults in the course of conversation with customers. This can only lessen the chances to sell a stone which is not loupe clean. Inclusions are not faults, but features which creative nature has drawn in numerous ways.
| Illustrations P3 | Models of Microscopes |