296 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



Twin-crystals of spinel are common. In Fig. 60 d, the two individuals are united in 

 such a manner that one face of the octahedron is common to both, and the two individuals 

 are svmmetrical about this face. The other octahedral faces form alternate salient and 

 re-entrant angles at the plane of junction. The twinning together of two octahedra in this 

 manner is of such frequent occurrence in spinel that such compound forms, which occur in 

 several other minerals, including the diamond, are said to be twinned according to the 

 spinel-law. The two individuals of a spinel-twin have often very little thickness in the 

 direction perpendicular to the common octahedral face. The crystal, then, has the 

 form of a triangular plate with alternate salient and re-entrant angles at the edges. 

 One of the individuals of the twin may be twinned in the same way with a third 

 octahedron, and this again with a fourth, and so on. This may give rise to very com- 

 plicated groups. 



Spinel has no cleavage, or, if any, a very imperfect one, and its fracture is irregular to 

 conchoidal. The mineral is brittle and hard, being the fourth hardest precious stone 

 known ; it immediately follows chrysoberyl, and is very little harder than topaz, the number of 

 which, on Mohs' scale of hardness, is 8. The specific gi-avity is fairly high (sp. gr. = 3.60 to 

 3-63), not far removed from that of diamond. Spinel becomes positively electrified when 

 rubbed, but acquires no pyroelectrical charge when heated or cooled. It is unattacked by 

 acids, and infusible before the blowpipe. 



The lustre of spinel, though of the common vitreous type, is very brilliant, especially on 

 polished facets. It is suceptible of a very high degree of polish, not, however, equalling the 

 ruby in this respect. It is stated to be possible for an expert to distinguish a ruby from a 

 spinel by the difference in brilliancy of lustre alone. Some spinels are beautifully clear and 

 transparent, while others are cloudy and opaque, the latter being, of course, valueless as gems. 

 All varieties of spinel, since they crystallise in the cubic system, are singly refracting, that 

 is to say, optically isotropic. Thus the polariscope offers a simple and ready means for 

 distinguishing a spinel from a ruby. The refractive power of spinel is fairly high, being 

 about the same as that of corundum. The refractive indices for different colours of the 

 spectrum do not difi^er widely ; they were determined for a pale red spinel as follows : 



n = 1'71 for red light. 

 n = 1*72 for yellow light. 

 n = 1-73 for kue light. 



The dispersion, as in the ruby, is therefore small, and the cut gem produces no marked display 

 of prismatic colours, such as one is accustomed to see in the diamond. 



When absolutely pure the substance of Spinel is perfectly free from colour, and colourless 

 octohedra of the mineral have been found in nature, though rarely. Except in the matter 

 of lustre, such crystals will resemble octahedra of diamond very closely, both minerals, 

 moreover, being optically isotropic and of nearly the same specific gravity. They are very 

 easily distinguished, however, by the great difference in hardness which exists between them. 

 Dufrenoy stated the weight of a perfectly colourless spinel, which came from India in a cut 

 condition, to be 12"641 grams (61| carats). 



Precious spinel is commonly, however, red in colour, of shades which incline to violet 

 and blue on the one hand and yellow on the other. It is stated that in all red spinels, 

 whatever be their shade of colour, a tinge of yellow reflected from the interior of the stone, 

 especially if it be cut, can be detected, by which they can be distinguished from the 

 ruby. Spinel being optically isotropic is not dichroic, so that, in contradistinction to the 

 ruby, it appears of the same colour in whatever direction it is viewed. Specimens of spinel 



