336 syste;matic description of phecious stones 



south-west to north-east direction from Capao de Lana through Joao Correa and Boa Vista 

 to Ouro Preto. The deposit has been traced as far south as Chiqueiro d'Aleinao. 



The topaz is here confined to a band, a few hundred yards wide, which extends with 

 little interruption throughout the range of hills in the direction mentioned. This is the 

 primary situation of the stone, that is to say, the place where it was formed. Water-worn 

 pebbles of yellow topaz are rare, but have been met with in the " tapanhoacanga " of the 

 neighbouring streams, a term which was explained in connection with the occurrence of 

 diamond in Brazil. Although the topaz now lies unchanged at the place where it was 

 originally formed, yet the rock in which it occurs has undergone considerable decomposition 

 and alteration. These rocks consist of clay-slates associated with itacolumite. They are 

 penetrated by quartz veins, and it is probable that the topaz, like the diamonds of 

 Diamantina, was formed in cavities in these quartz veins. The clay-slates are altered in 

 places by weathering into a soft clayey mass, and at isolated spots in this mass are found 

 druses or nests, containing detached topaz crystals embedded in a clay or scaly kaolin, 

 which ranges in colour from white to dark brown. These nests are probably isolated 

 portions of the quartz veins by which the rock was penetrated and in which the topaz 

 ■crystals were formed ; crystals are also found, however, in the clayey mass itself. 



The minerals associated with topaz are practically the same as have been frequently 

 observed to occur with diamond in the quartz veins which intersect the itacolumite and 

 accompanying rocks. Besides quartz (rock-crystal and smoky-quartz) they include ilmenite, 

 hsematite, rutile, black tourmaline, and the rare euclase ; all occur in broken fragments 

 embedded in the clay. It should, however, be noted that euclase is never found actually 

 with topaz, but always in a druse by itself. 



A different opinion as to the origin of yellow topaz in Brazil has been recently (1901) 

 expressed by Orville A. Derby. He states that it occurs near Ouro Preto, usually in 

 nodules in a clayey matrix which has resulted from the decomposition of a mica-schist. 

 This latter, he considers, has been formed by the metamorphism of an igneous rock 

 of the augite- or nepheline-syenite group, in the drusy cavities of which topaz had 

 crystallised out. 



The mode of occurrence of topaz at Saramenha, half an hour's journey from Ouro Preto, 

 is somewhat different. Here the crystals are embedded in a deposit of brown iron-stone 

 (limonite) intermixed with micaceous iron-ore (haematite), in which, after removal, they 

 leave sharp, bright impi-essions. The mineral is here pale yellow in colour and abundant in 

 quantity. 



The topaz which occurs in the neighbourhood of Ouro Preto varies in shade from a 

 pale yellow to a dark wine-yellow. As a general rule, the darker the kaolin in which the 

 topaz crystals are embedded the darker are these crystals. A finely coloured crystal from 

 this district is represented in Plate XIII., Fig. 2, and a faceted stone in Fig. Qa, of the 

 same plate. The most beautiful of the stones found in this district are those having the 

 rich colour of old Malaga wine. Red crystals (Plate XIII., Fig. 4,) also occur ; they are 

 usually of a pale rose shade, but may be a dark ruby-red, when they are known as 

 " Brazilian ruby," a term which is also applied to stones the red colour of which has been 

 produced by artificial means. A faceted " Brazilian ruby " is represented in Plate XIII., 

 Fig. ia ; the stone is highly prized by connoisseurs. 



The degree of transparency varies in different specimens, and the crystals are not all of 

 gem-quality ; thus among a thousand stones there may perhaps be but one perfect example, 

 all the rest being faulty in some way or another. 



With regard to size, crystals with a length of 6, or even of 10 inches, and a thickness 



