MINERALOGY. 427 



beautiful as love. It presents opalescent internal reflec- 

 tions of the most beautiful and diversified colors. It 

 occurs in irregular veins or detached masses among por- 

 phyry, and is highly valued as a gem. The handsomest 

 opals are those found in Hungary. 



Common Opal Semi-Opal has a greasy luster or 

 translucence ; structure firm, compact, and marbled ; is 

 slightly resinous, and although mostly white, is of vari- 

 ous colors, as gray, yellow, and green, seldom red ; 

 occurs in serpentine and basalt; is most frequent in 

 Hungary, Saxony, and Silesia. It receives its name of 

 Semi-Opal on account of its duller colors and being less 

 translucent. 



II. Diamond. Diamonds are distinguished for their 

 perfect transparency, their vitreous electricity when rubbed 

 or placed in the sun, and their brilliant reflections of 

 light and adamantine luster. The composition is pure 

 carbon, but on account of its external resemblance to the 

 Silicia genus, it is here placed among the minerals which 

 compose that species. It burns and is consumed at a 

 high temperature, producing carbonic acid gas, II. = 10 

 as the hardest body, G. = 3.4 to 3.6. Diamonds occur 

 chiefly in alluvial deposits of gravel and sand, lying in 

 detached crystals, sometimes with plain but more fre- 

 quently with rounded surfaces. They were originally 

 discovered in Bengal, but they have since been found in 

 the East Indies, Brazil, and the Ural Mountains. The 

 perfectly pure diamond is as transparent as water, in 

 which state it is known as a " diamond of the first water," 

 and commands a higher or lower price in proportion as it 

 falls short of this perfection. Those that are colored 

 blue, green, red, gray, or yellow, are less esteemed than 

 the transparent, which, being considered the most orna- 



