56 



JOURNAL OF THE 



[October, 



mixture. Some few were observed inclosing inner bubbles, 

 apparently a double cavity, but empty. In natural rubies, the 

 cavities are always angular or crystalline in outline, and are 

 usually filled with some liquid, or, if they form part of a 

 " feather" as it is called by the jewelers, they are often arranged 

 with the lines of growth. Hence the difference in appearance 

 between the cavities in the natural gem and those in the fused 

 gem is very great, and can readily be detected by the pocket 



Fig. 1.— Spherical cavities in artificial Fig, 2.— Spherical and irregular cavities 



ruby as seen at one time (enlarged in artificial ruby as seen at one time, 



75 diameters). evidently from the lower part of the 



crucible (enlarged 25 diameters). 



lens. I have failed to find in any of the artificial stones even a 

 trace of anything like a crystalline or angular cavity. Another 

 distinguishing characteristic is that in many genuine rubies we 

 find a silky structure (called "silk" by the jewelers), which, if 

 examined under the microscope, or under a A to ys inch objec- 

 tive, 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 sufficient number, these acicular and 

 arrow-shaped crystals produce the asteria or star effect, if the 

 gem is cut en cabochon form with the centre of the hexagonal 



Fig. 3— Liquid cavities in natural ruby and sapphire (enlarged 100 diameters). 



prism on the top of the cabochon. I have failed to find any of 

 them in the stones under consideration, or even any of the 

 marking of the hexagonal crystal which can often be seen when 

 a gem is held in a good light, and the light allowed to strike 

 obliquely across the hexagonal prism. Dr. Isaac Lea has sug- 

 gested^ that these acicular crystals are rutile, and interesting 

 facts and illustrations have been published by him. From my 



> Proc. Philad. Acad. Sc, Feb. 16, 1669, and May, 1876. 



