300 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



CHRYSOBERYL. 



In contrast to the variety in colour presented by corundum, a species which has 

 furnished the jeweller with an extensive range of precious stones of identical composition, 

 but dissimilar appearance, the mineral we have now to consider, namely, chrysoberyl, is 

 characterised by the absence of any great range of colour, and displays only a few tints of 

 green and its closely-related colour, yellow. As the consideration of the mineral spinel was 

 appropriately made immediately to follow that of corundum, on account of the similarity 

 in the coloiu-s of the two minerals, so now the next place is given to chrysoberyl on account 

 of its hardness approximating to that of corundum. 



The hardness of chrysoberyl is exceeded only by that of corundum and, of course, of 

 diamond. In Mohs' scale of hardness it is placed between corundum (9) and topaz (8), and 

 has a hardness of 8^ assigned to it, being, therefore, the third hardest mineral known. 



Like the other extremely hard minerals, corundum and spinel, chrysoberyl is composed 

 very largely of alumina, containing 80-2 per cent, of this oxide and 19-8 per cent, of 

 beryllia, a compound of oxygen with the metal beryllium, in the same way as alumina is 

 a compound of oxygen wibh the metal aluminium. This percentage chemical composition 

 corresponds to the chemical formula BeO.AljOg. Chrysoberyl is, however, never found 

 in nature in this ideally pure condition; most of the material which has been analysed 

 contains iron in small amounts, while in alexandrite, a variety of chrysoberyl found in the 

 Urals, a little chromic oxide is also present. The following are typical analyses of ordinary 

 chrysoberyl from Brazil and of alexandrite from the Urals : 



100-92 100-78 



Chrysoberyl crystallises in the rhombic system. Simple crystals are rare, more or less 

 complicated twin-crystals being most frequently met with ; the former have the form of 

 short, rhombic prisms in combination with other forms (Fig. 61 a). Both simple and 

 twinned crystals are tabular in the direction of a pair of parallel faces, which are striated as 

 indicated in the figure. Two crystals are frequently twinned together in such a way that 

 the resulting form is symmetrical about a plane perpendicular to the striated face, that is 

 to say, each individual of the twin is a reflection of the other in this plane, which is called 

 the twin-plane. In the compound crystal the striated faces of the two individuals are 

 co-planar, and the two sets of striations meet at the twin junction at an angle of very 

 nearly 60° (Fig. 61 i, and Plate XII., Fig. 10). It frequently happens that three crystals 

 are twinned together according to this same law, and when they also interpenetrate a very 

 complicated group results (Fig. 61 c, and Plate XII., Fig. 8). In such compound forms the 

 boundaries of the individual crystals may be easily traced by the intersection of the three 

 sets of striations at angles of approximately 60°. 



