guishiug characteristic is that in many genuine rubies we flud a silky structure (called " silk" by the jewelers), which, if examined under the microscope, or under a T4G to Tpy inch objective, we find to be a series of cuneiform or acicular crystals, often iridescent, and arranged parallel with the hexagonal layers of the crystal. When in suflicieut number, these acicular and arrow-shaped crystals produce theasteriaor star effect, if the gem is cut en cabochon form with the center of the hexagonal prism on the top of the cabochou. I have failed to find any of them in the stones under consideration, or even any of the markings of the hexagonal crystal which ean often be seen when a gem is held in a good light, andthelight allowed to strike obliquely across the hexagonal prism. Dr. Isaac Lea has suggested (a) that these acicular crystals are r utile, and interesting facts and illustrations have been published by him. From my own observations on many specimens, I believe there is little doubt of the truth of this hypothesis. My explanation is that they were deposited from a solution, either heated or cold, while the corundum was crystallizing, and I doubt very much whether they will ever be found in any substance formed by fusion. The hardness of these stones was found to be about the same as that of the true ruby, 8.8, or a trifle less than 9, the only difference being that the artificial stones were a trifle more brittle. The testing-point used was a Siamese green sapphire, and tbe scratch made by it was a little broader, but no deeper than on a true ruby, as is usually the case with a brittle material. After several trials it was faintly scratched with chrysoberyl, which will also slightly mark the true ruby.
The specific gravity of these stones was found to be 3.93 and 3.95. The true ruby ranging from 3.93 to 4.01, it will be seen that the difference is very slight and due doubtless to the presence of tbe included bubbles in tbe artificial stones, which would slightly decrease the density. As a test, this is too delicate for jewelers' use ; for if a true ruby were not entirely clean, or a few of the bubbles that sometimes settle on gems in taking specific gravities were allowed to remain undisturbed, it would have about the same specific gravity as one of these artificial stones.
I found on examination by the dichroscope that the ordinary image was cardinal red, and tbe extraordinary image a salmon red, as in tbe true ruby of the same color. Under tbe polariscope, what I believe to be annular rings were observed. With the spectroscope the red ruby line, somewhat similar to that in tbe true gem, is distinguishable, although perhaps a little nearer tbe dark end of tbe spectrum. The color of all tbe stones examined was good, but not one was so brilliant as a very fine ruby. The cabochons were all duller than fine, true stones, though better than poor ones. They did not differ much in color, however, and were evidently made by one exact process or at one time. Their dull appearance is evidently due in part to the bubbles. The cProc. Philad. Acad. Sc, Feb. 16, 1669, and May, 1876.
