Surface Finishes Of Gems
SURFACE FINISHES
Maximum brilliancy in any faceted gem is achieved only when the facets are cut as fiat as possible and polished to the highest degree of smoothness. The quality of surface finish is highly important in cabochon gems also, particularly in those displaying special optical effects such as star stones, cats eyes, and moonstones. A perfect polish also greatly enhances the play of color in opal.
But to return to faceted gems, it is well known that the flatness of facets and the perfection of polish is best achieved on the hardest gem-stones. Thus diamond, hardest of all, is capable of being cut and polished so perfectly that electron microscope photographs show scarcely any kind of surface irregularity even at magnifications of thousands of times. This superior flatness is not only attributable to the nature of the diamond itself, but also to the iron laps which are used for cutting and polishing. A careful diamond cutter keeps his laps in top condition so as to avoid any trace of irregularity on the facets caused by the grooves which appear on any kind of polishing lap after some use. Stones less hard than dia¬mond very commonly show minute parallel polished grooves, particularly upon the large table facet, and indicate that the cutter allowed the stone to rest too long on the same place on the lap. In contrast to these “polish marks,” as they are called, are the minute scorings with ragged edges which result from tearing of the surface of the gem. These tears may be due to small particles of grit caught in the surface of the lap or to minute fragments of the gem itself which have broken loose and ploughed through the facet under the force of the spinning lap.
Surface defects are most likely to appear upon the cheaper gemstones, where the rate of production must be high to keep lapidary charges low. Too much attention cannot be given to individual stones, and this usually shows up when the facet surfaces are examined with low-power magnification under a good light. Synthetic ruby, sapphire, and spinel often show a series of very small cracks on facet junctions which are due to the relief of stresses within the material created by the method of manufacture. They are commonly called “fire marks” because it is be¬lieved that they arise from polishing that is too rapid, and, hence, too hot. Many of these cracks are so small that a magnification of about 20x or more is required to see them well.
While “fire marks” and other surface defects may not be readily visible to the naked eye, they nevertheless cause scattering of light from the sur¬faces of the gem and decrease the total brilliance that could have other¬wise been developed.
Softer gemstones, particularly those which are cut only for collectors, present many problems in polishing and are those most likely to show polishing defects. Some of them cannot be polished well except on soft and yielding laps, with the result that the facet junctions are noticeably rounded. While a good brilliance is obtained, the reflection usually lacks the crispness of well-cut gemstones of greater hardness.
Aside from the exceptions noted for soft stones, well-cut and well-polished faceted gems should display good proportions, flat facets of regular shape and all of the same size in any particular row of like facets, accurate junctions between facets without overlaps or gaps and a smooth, glassy polish, particularly on the large and highly important table facet. Scratches and polishing marks should not be visible to the naked eye. In the diamond trade, where quality grades are strictly defined, none of these defects should be visible under lOx magnification. Additional diamond trade practice rules, as promulgated by the U.S. Federal Trade Com¬mission, are furnished under the Diamond listing.