30 GENERAL CHARACTERS OF PRECIOUS STONES 



the stone which is scratched is the softer of the two, and if neither is scratched the stones 

 are of equal hardness. 



In this way it can be shown that all specimens of the same mineral species have the same 

 degree of hardness, and that this quality will differ more or less considerably in different 

 minerals. The hardness of a mineral is, therefore, one of its characteristic features, and it 

 affords a means whereby precious stones may be identified or distinguished. 



From observations on the relative hardness of minerals made in the way just described, 

 all minerals can be arranged according to the degrees of hardness they possess in a series 

 ranging from the softest to the hardest. From such a series, the late Viennese mineralogist, 

 Mohs, selected the hardest, the softest, and eight minerals of intermediate hardness, and with 

 them constructed a table for use as a scale of hardness. The ten selected minerals were 

 numbered consecutively from 1 to 10, No. 1 being the softest and No. 10 the hardest. 

 The scale is given below : 



1. Talc. 6. Felspar. 



2. Gypsum. 7. Quartz. 

 .S. Calcito. 8. Topaz. 



4. Fluor-spar. 9. Corundum. 



5. Apatite. 10. Diamond. 



It must be borne in mind that the difference in hardness between any two consecutive 

 members of this series is by no means identical. Thus, for example, the difference in hardness 

 between diamond (10) and corundum (9) is vastly greater than between corundum (9) anil 

 topaz (8) ; greater indeed than the difference which exists between corundum (9) and talc 

 (1). The different minerals in this scale of hardness are chosen solely with a view to practical 

 convenience in mineralogy, namely to afford a means whereby the relative hardness of any 

 mineral may be expressed with clearness and brevity by a number. 



To express the hardness of any given stone in this way we must first ascertain to which 

 member of the scale it corresponds in hardness ; the number of this member then expresses 

 the hardness of the stone, which may be written H = 8, for example. By this it is under- 

 stood that the hardness of the stone corresponds to No. 8 on Mohs' scale, that is, it has the 

 same hardness as topaz. In the same way, if the hardness of the stone lies between that of 

 quartz and of topaz, H = 7 — 8 ; should it be nearer quartz, then H = 7^ ; or nearer topaz, 

 then H = 7f ; while if it lies apparently midway between them, H = 7^. In the last case 

 the stone would scratch quartz with the same ease as it is itself scratched by topaz. H = 7{ 

 means that topaz scratches the stone more easily than the stone itself scratches quartz ; 

 while H = 7| means that the stone easily scratches quartz but is not so easily scratched by 

 topaz. Beyond this these numbers have no exact meaning. 



Specimens of the ten minerals which form the scale of hardness should be kept specially 

 for the purpose of determining hardness. They should be crystals with sharp edges and 

 smooth faces, and of a convenient size. In testing any given stone a sharp edge of the scale 

 mineral is rubbed over a smooth face of the stone, the scale minerals being used in consecu- 

 tive order from the softest to the hardest. It is important to distinguish carefully between 

 a scratch on the surface of the stone undergoing examination and a streak of powder which 

 may arise from abrasion of the corner of the scale mineral. To avoid this mistake the surface 

 should be wiped and then examined with a lens. 



To take an example, let us suppose that the stone to be tested is not scratched by any 

 member of the scale until topaz is tried ; its hardness will then lie between 7 and 8. If it 

 is not capable of itself scratching quartz its hardness is exactly that of quartz, namely, 7. 

 Should the quartz, however, be scratched by the stone, its hardness is then H = 7 — 8, or, as 



