DIOPTASE 



419 



size and beauty as at the locality just mentioned. In America stones similar to those from 

 the Knappenwand have been found at Rabun Gap, in Rabun County, Georgia ; and fine 

 dark green crystals have come from Roseville, in Sussex County, New Jersey, and from 

 Haddam, in Connecticut ; they are only occasionally, however, cut as gems. Again, a few 

 crystals of green epidote are found with green tourmaline in Brazil. There must be 

 many stones suitable for cutting to be found at the numerous loeahties for epidote, but 

 since the use of this mineral as a gem is so limited and its value is so small, search for them 

 is scarcely likely to be made. 



PIEDMONTITE. 



Piedmontite is a variety of epidote, in the composition of which alumina is replaced by 

 manganese sesquioxide. This manganese-epidote is found principally in the manganese 

 mines of San Marcello in the Piedmontese Alps, from whence the name piedmontite is 

 derived. It sometimes occurs in magnificent cherry-red crystals, which, when they are of 

 sufficient transparency, are cut as gems. 



DIOPTASE. 



Dioptase is a mineral of a deep green colour approaching, but always darker than, that 

 of emerald. The two stones differ widely in chemical composition, however, for dioptase is 

 a hydrous silicate of copper with the formula HgO.CuO.SiOg. The results of an analysis 

 of a crystal from the best and longest known locality, namely, that in Siberia, are given 



below : 



Per cent. 



36-60 



48-89 



2-00 



12-29 



Silica (SiOa) . 

 Cupric oxide (CuO) . 

 Ferrous oxide (FeO) 

 Water (HjO) . 



99-78 



The green colour of dioptase is due to the large amount of copper which is present, and 

 for the same reason the mineral is sometimes referred to as emerald- 

 copper. 



Dioptase occurs usually in well-developed but small crystals. These 

 are rarely larger than a pea, and belong, like crystals of emerald, to the 

 hexagonal system. The six-sided prisms of dioptase, however, are termi- 

 nated by the faces of a rhombohedron, and in rare cases the alternate 

 edges between the striated rhombohedron-faces and the smooth prism- 

 faces are replaced by narrow faces indicating a tetratohedral development 

 of the crystal. This is shown in Fig. 79, which represents a Siberian 

 crystal. There is a distinct cleavage parallel to the faces of a rhombo- 

 hedron which truncate the polar edges of the primitive rhombohedron 

 represented in the figure. On these edges there is therefore sometimes 

 to be seen a pearly lustre, but otherwise the lustre of the crystals is of 

 the common vitreous type. Dioptase is not very hard ; it is scratched even by felspar and 

 is itself scarcely capable of scratching glass, so that its hardness is approximately 



Fig. 79. Crystalline 

 form of dioptase. 



