sm 



SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



Topaz crystallises in the rhombic system ; all crystals have certain features in common, 

 but show differences in habit. A combination of two rhombic prisms forming elongated, 

 eight-sided columns, often deeply striated parallel to their length, is almost invariably to be 

 seen. The terminal faces differ according to the locality from which the crystal comes. As 

 a rule, they are developed regularly at one end only of the crystal, the other end having been 

 attached to the matrix in the drusy cavity in which the crystal grew. A few forms taken 

 by topaz crystals are shown in Pig. 66, a to d. 



b. c. 



Fig. 66. Crystalline forms of topaz. 



d. 



Pig. 66 «, and Plate XIII., Figs. 2 and 4, show a simple form of crystal especially 

 characteristic of topaz from Brazil and Asia Minor ; here the only terminal faces are those 

 of a rhombic octahedron or pyramid. In Pig. 66 b, the faces of this rhombic octahedron 

 are small, and two large dome faces, give a roof-like termination to the crystal ; this habit 

 is characteristic of topaz crystals from the Adun-Chalon Mountains, near Nerchinsk, in 

 Siberia. Crystals from Mursinka, in the Urals (Fig. QQ c, and Plate XIII., Fig. 1), have, 

 in addition to these faces, a largely developed basal plane at right angles to the prism faces. 

 The crystal shown in Fig. QQ d, is a combination of two prisms, three rhombic octahedra, a 

 dome, and the basal plane ; this habit is characteristic of crystals from Schneckenstein in 

 Saxony (Plate XIII., Fig. 3). More complicated crystals, with a much larger number of 

 faces, are to be found at other localities ; the examples cited are sufficient, however, to give 

 a general idea of the crystalline forms of topaz. 



Crystals of topaz as small as a pin's head have been found, but very large ones weighing 

 several pounds are not at all unusual. Thus, for example, a beautiful transpai'ent crystal of 

 topaz, weighing more than 25 pounds, was found in the neighbourhood of the Urulga river in 

 Siberia. A crystal of topaz, 2 feet in length and 137 pounds in weight, has been found quite 

 recently (1901) in Saetersdalen, Norway, and is now exhibited in the British Museum. 



Topaz differs from the majority of precious stones in the possession of a very perfect 

 cleavage. There is only one direction of cleavage, and this is parallel to the basal plane — 

 that is to say, at right angles to the length of the striated prism. In consequence of this 

 cleavage, topaz crystals, when removed from the matrix, almost invariably break away with 

 a smooth, shining, plane face ; the lower ends of the crystals in Pig. 66 are terminated by 

 such cleavage planes. A crystal of topaz, which is too long to be cut as a single stone, may 

 be readily cleaved with a chisel into fragments of suitable size, and thus much laborious 

 work avoided. The drawback connected with this pei'fect cleavage is that it is the cause 

 of a tendency in the stone to develop plane, even fissures. The presence of such fissures, 

 which is often indicated by brilliant iridescent colours, detracts considerably from the 

 beauty and vahie of the stone. To avoid the development of these fissures, the stone must 



