THE GARNET GROUl' S45 



■distinguished by its greater hardness (H = 9) and lower specific gravity (sp. gr. = 4-0). The 

 two stones differ also in another respect ; for " oriental hyacinth " is distinctly, though feebly, 

 'dichroic, while the dichroism of true hyacinth is so feeble as to be scarcely observable. 



The name hyacinth is sometimes also applied to rod crystals of ferruginous quart/ of 

 'the variety known as "Eisenkiesel," which occurs, for example, embedded in gypsum at 

 Santiago de Compostela in the north of Spain, and is known as " Compostela hyacinth." It 

 IS much inferior to true hyacinth both in lustre and transparency ; moreover, being quartz, 

 its specific gravity is only ^-65 and it floats in methylene iodide. Some of the colour-varieties 

 •of tourmaline are somewhat similar in appearance to certain zircons, less often, however, to 

 ihyacinth. They may be readily distinguished, however, for tourmaline is strongly dichroic, 

 ■and being much lighter than zircon floats in methylene iodide. Rose-topaz is also 

 sometimes substituted for hyacinth ; here again the former is of quite a different shade of 

 ■colour and is strongly dichroic. 



Imitations of hyacinth in coloured glass may be easily distinguished fi-om genuine 

 stones by the fact that they are singly refracting and much less hard. 



THE GARNET GROUP. 



Garnet is extensively used as a gem, and is to be found adorning some of the costliest 

 as well as the simplest articles of jewellery. If the ordinary display in a jeweller's window is 

 •observed, it will often be found that at least one half of the gems exposed for sale are garnets 

 ■of various kinds. 



Garnet differs from diamond, corundum, and many other precious stones hitherto 

 •considered in that its chemical composition is not fixed and unchangeable, but is subject to 

 •considerable variation. The minerals grouped together as garnet have some physical 

 •characters in common ; they all have the same crystalline form and the same type of 

 -chemical constitution. The several members of the group differ from each other, however, 

 in the chemical elements of which they are composed ; the garnets, in fact, form what is 

 •called an isomorphous series, such as is often met with in the mineral kingdom. 



The point of agreement in the chemical composition of all garnets is the association of 

 three molecules of silica (SiOg) with one molecule of a sesquioxide, represented generally by 

 RgOg, and with three molecules of a monoxide, represented by MO. The chemical formula 

 which expresses the composition of the whole garnet group in general terms is therefore 

 SMO.RgOg.SSiOj. The various members of the group differ from each other, howevei-, in 

 the nature of the monoxide and sesquioxide which take part in their constitution ; thus the 

 monoxide may be lime (CaO), ferrous oxide (FeO), or magnesia (MgO), occasionally also 

 manganous oxide (MnO), or chromous oxide (CrO), while the sesquioxide may be alumina 

 •{AlgOj), or ferric oxide (FegOg), or sometimes chromic oxide (Ci-oOg). 



These different monoxides and sesquioxides are associated together in the most varied 

 proportions, always, however, conforming to the general formula given above. All the 

 varieties of garnet, which it is theoretically possible to construct by different combinations 

 ■of the oxides mentioned above, are not actually known to exist in nature ; the small number, 



