A Text-Book of Precious Stones for Jewelers and the Gem-Loving Public

by Frank Bertram Wade

The matter being thus introduced, the lady was at once informed that even a real emerald might show signs of wear after a few years of the hard use that comes to a ring stone.

While emerald has, as we saw in the lesson on hardness, a degree of hardness rated as nearly 8 (71⁄2 in the table), it is nevertheless a rather brittle material and the long series of tiny blows that a ring stone is bound to meet with will cause minute yielding along the exposed edges and corners of the top facets. This being announced, the first step in the examination of the stone was to clean it and to give it a careful examination with a ten-power lens. (An aplanatic triplet will be found best for this purpose.)

Color. The color was, of course, the most obvious property, but, as has already been said, color is not to be relied upon in all cases. In this case the color was a good emerald green but a bit bluer than the finest grass green. A very fine Maine tourmaline might approach this stone in color, so it became necessary to consider this possibility. A glass imitation, too, might have a color equal or superior to this.

Imperfections. While noting the color, the imperfections of the stone claimed attention. They consisted mainly of minute jagged cracks of the character peculiar to brittle materials such as both emerald and tourmaline. So far it will be noted either of the above minerals might have furnished the lady's gem. As glass can be artificially crackled to produce similar flaws the stone might have been only an imitation as far as anything yet learned about it goes.

File Test. The next step was to test its hardness by gently applying a very fine file to an exposed point at one corner of the girdle. The file slipped on the material as a skate slips on ice. Evidently we did not have to do with a glass imitation.

Refraction. Knowing now that we had a true hard mineral, it remained to be determined what mineral it was. On holding the stone in direct sunlight and reflecting the light onto a white card it was seen at once that the material was doubly refracting, for a series of double images of the back facets appeared. These double images might have been produced by tourmaline as well as by emerald. (Not however by glass which is singly refracting.) If a direct reading refractometer had been available the matter could have been settled at once by reading the refractive indices of the material, for tourmaline and emerald have not only different refractive indices but have double refraction to different degrees. Such an instrument was not available at the time and will hardly be available to most of those who are studying this lesson, so we can go on with our account of the further testing of the green stone.

Hardness. A test upon the surface of a quartz crystal showed that the stone was harder than quartz (but so is tourmaline). A true topaz crystal was too hard for the ring stone, whose edge slipped over the smooth topaz surface. The green stone was therefore not a green corundum (Oriental emerald) as the latter has hardness 9 and scratches topaz.

With hardness evidently between 7 and 8 and with double refraction and with the kind of flaws peculiar to rather brittle minerals we had in all probability either a tourmaline or an emerald.

Dichroism. The dichroscope (which might have been used much earlier in the test but was not at hand at the time) was next tried and the stone was seen to have marked dichroism—a bluish green and a yellowish green appearing in the two squares of the instrument when the stone was held in front of the opening and viewed against a strong light.

As either tourmaline or emerald might thus exhibit dichroism (the tourmaline more strongly, however, than the emerald) one more test was tried to finally decide the matter.