NATURAL SCIENCES OF PHILADELPHIA. 



119 



Further Notes on MICROSCOPIC CRYSTALS in some of the Gems, fee. 

 BY ISAAC LEA. 



In a paper which I recently read to the Academy, I mentioned having found 

 acicular crystals in Precious Garnets. Since then I have had the opportunity of 

 examining a number of cut specimens o^ Sapphire in the forms oi Aster ia, Catseye, 

 &c. I have also examined many specimens of Cinnamon Stone from Ceylon, 

 brought by Dr. Ruschenberger, of the United States Navy, also, among others, 

 a very fine specimen of bluish Sapphire, in tlie collection of Prof. Leidy. 



Having made microscopic drawings of these and other species, having in- 

 cluded microscopic crystals, I propose to present them with as nearly correct 

 illustrations as possible. 



The whole subject of microscopic mineralogy has been of great interest to 

 me, and I hope these short notes may induce some student to pursue the sub- 

 ject to a greater extent than I have had it in my power to do. It cannot fail 

 that, with the use of the numerous admirable microscopes now made in this 

 country, working with so much more facility than with those we have been 

 accustomed to from abroad, observers may continue to bring to our know- 

 ledge much that has been heretefore unknown and very little suspected in this 

 branch of science. 



In my former paper I stated the proportional number among Bohemian Gar- 

 nets vrhich I found to contain microscopic crystals. I now propose to give 

 descriptions and figures of the appearance of these crystaline forms, and with 

 this view I have made drawings of their apparent forms under a power of 

 about 100 diameters. 



Sapphire. A very remarkably beautiful Asteriated bluish Sapphire, procured by 

 Dr. Ruschenberger when in Ceylon, presented to the naked eye the six rays 

 which in the sun were sharp and of great beauty. The specimen being set as 

 a gem of luxury, I could not get a view by transmitted light, but by reflected 

 light, with great care, the exceedingly minute crystals were distinctly seen. 

 They are very short, of pearly lustre, at three different equal angles, thus pro- 

 ducing the bands which form the rays in three directions of 60° each. The 

 reflection from the sides of these minute crystals cause, of course, the asterism of 

 six rays over any point of the curved polished surface of the specimen. These 

 rays are formed on the same principal precisely as the asterism in Phlogopite, 

 which I have mentioned elsewhere. 



Fig. 1 represents the delicate, numerous, minute crystals in the beautiful 

 .4s<e7-ia referred to above belonging to Dr. Ruschenberger. The acicular crys- 

 tals are so small that it was with great difiSculty I obtained their position as 

 here represented. 



The variety of Sapphire (Corundum) which goes under the name of Catseye, 

 has irregular coarse stria;, which have the appearance of being Asbestus as is 

 generally supposed. In this gem there is a single band which varies accor- 

 ding to the position it may be placed in, and by no means has the beauty of 

 the asteriated Sapphire. Several of these are now before me which came 

 from Ceylon. 



Fig. 2 represents the crystals which I observed in a fine small bluish Sap- 

 phire, in Prof. Leidy's fine collection of gems. The cuneiform or arrow-headed 

 crystals are very extraordinary, and they may be simply twin crystals of some 

 substance of which at present we can have no perfect idea. They remind us 

 in their form of Selenite crystals, such as are found in the Paris Basin, and at 

 once we recognise the similarity to the cuneiform character stamped on the 

 bricks of Babylon, and cut in the alabaster monuments of Nineveh. The group 

 which I have drawn represents six of these cuneiform crystals, and six acicu- 

 lar crystals. Of the former six, four had a bluish tinge and two were pinkish. 

 The acicular crystals were disposed to take three different directions, parallel 



1869.] 



