SPINEL 295 



SPINEL. 



The precious stones which are most appropriately considered after corundum are those 

 belonging to the spinel group of minerals. Their colour often resembles that of the ruby, 

 but in all other characters the two minerals are perfectly distinct, so that the names 

 " ruby-spinel " and " balas-ruby,'" which are sometimes given to certain colour-varieties of 

 spinel, are misleading and incorrect. Spinels of a sapphire-blue colour are also known, but, 

 like black spinels, they are of little importance. 



In scientific mineralogy the spinel group includes a very large number of minerals of 

 varied composition but of identical crystalline form and chemical constitution. Of all these 

 minerals of the spinel group, differing widely from each other in chemical composition, 

 hardness, colour, transparency, &c., there is but one which is generally used as a gem, and 

 this particular stone is therefore distinguished as precious spinel, or noble spinel. 



Precious spinel is a compound of alumina (the sole constituent of ruby) with magnesia, 

 its composition being represented by the formula MgO.AljOg or MgAlgO^. This compound 

 is in itself colourless, so that the various colour-varieties of spinel owe their tints to the 

 presence of small quantities of foreign substances. The colour both of the ruby and of 

 the red spinel is thought to be due to the presence of chromic oxide (CrgOg), but, according 

 to Doelter, the presence of the small amount of iron, as shown in the following analysis, is 

 sufficient to account for the colour of these stones. The analysis referred to was made by 

 Abich on a red spinel from Ceylon. 



Alumina (AljOj) . . . 7U'43 per cent. 



Chromic oxide (CrgOs) . . . 1'12 ,, 



Magnesia (MgO) . . 26-75 „ 



Ferrous oxide (FeO) . . . . 0-73 „ 



Spinel crystallises in the cubic system, and the form of the crystals can often easily be 

 made out, even when they are very much rounded and water-worn. This form is most 



Fig. 60. Crystalline forms of spinel. 



frequently an octahedron (Fig. 60 a), uncombined with other forms and usually with faces 

 developed on all sides. Fig. 60 c illustrates the truncation of the edges of the 

 octahedron by faces of the rhombic dodecahedron, the result of which is a not uncommon 

 form. A less frequent form, except in the case of black spinel, is shown in Fig. 60 6, 

 in which each of the corners of the octahedron are replaced by four faces of an 

 icositetrahedron. 



