410 



SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



Bodenmais in Bavaria. Irregular masses of material, which, in part, is of a fine blue colour 

 and is sometimes transparent, occur in gneiss at Arendal, Kragero, Tvedestrand, and other 

 places in Norway, at Orijarfvi, near Abo, in Finland, and elsewhere. Fine stones are found 

 in granite veins in gneiss at Haddam in Connecticut, U.S.A. The pebbles found in Ceylon 

 are sometimes as large as a nut ; they occur with other precious stones in the gem-gravels 

 of the island. Cordierite is also said to occur, together with blue and white topaz, in the 

 district of Minas Novas in Brazil. 



IDOCRASE. 



Because of its occurrence on Mount Vesuvius idocrase is frequently referred to as 

 vesuvian or vesuvianite. It occurs at this locality in remarkably fine transparent brown 

 crystals, which are sometimes cut at Naples as gems, and, on this account, are known in the 

 trade as " Vesuvian gems." The use of this stone as a gem is not extensive, and is mainly 

 confined to Italy. Crystals of gem-quality and of a green colour are found also in the Ala 

 valley in the Piedmontese Alps ; a small number find their way into the gem markets 

 through the neighbouring town of Turin. Idocrase from other sources is scarcely ever cut 

 as a gem, so that it may be regarded as an Italian precious stone. 



Chemically, idocrase is a calcium-aluminium silicate containing small amounts of 

 water, iron oxides, and other constituents. The chemical composition was formerly 

 considered to be the same as that of calcium-aluminium garnet ; this, however, has been 

 shown to be incorrect, and the composition is now expressed by the complex formula 

 2H2O.12CaO.3(Al,Fe)2O3.10SiO2. Analyses of crystals from the two Italian localities 

 mentioned above have given the following results : 



100-45 



100-81 



Idocrase crystallises in the tetragonal system ; well-developed crystals are of frequent 

 occurrence. They occur usually attached to the walls of cavities and crevices in rocks, and 

 form beautiful druses. The forms taken by two crystals of idocrase are represented in 

 Figs. 73« and b ; the former represents a crystal from the Ala valley, and the latter one 

 from Vesuvius. 



The crystals in almost all cases take the form of prisms of greater or less length, with 

 four, eight, or more prism-faces, which are distinctly striated parallel to the edges of the 



