82 MINERALOGY. 
abounds. A large number of silicates would be of great value if found in 
vast quantities. The same also is true of the metals. Platinum, considering 
its properties, would be the most valuable metal for the most varied 
technical applications, did it only occur as abundantly as iron. Hence also 
it follows that our idea of the value of a mineral does not always express its 
utility. Limiting ourselves to well known species, we find single minerals 
of the first group, the metalloids, of very general utility ; of this, sulphur is 
an illustration. Even the rare variety of carbon, the diamond, is indispens- 
able in glass-working ; and on its employment as an ornamental gem is 
based a special branch of art, namely, diamond grinding, which is carried on 
in England and Holland. While the diamond is the hardest of all bodies, so 
its working is the most difficult. The hardness of the diamond exceeds that 
of other minerals so much, that no substance but its own dust can be used in 
grinding it. In grinding diamonds horizontal metal disks are used, which 
are made to rotate with immense velocity by means of a lathe. The 
diamond to be ground is cemented fast to a stem of wood or brass. The 
metal wheel is coated with oil, and strewn with the powder of impure or 
very small diamonds. Even the chips and fragments of the larger diamonds 
obtained are brought into use for the same purpose by pounding them in 
a steel mortar. After the wheel has been properly adjusted, the diamond is 
so fixed by means of the stem to which it has been cemented, that a certain 
part to be ground shall press upon the wheel, which is then to be set 
in motion. In this operation the diamond powder becomes finer, and 
the diamond is worn away by the attrition. After one face has been 
properly ground, the diamond is fixed in another position, and the operation 
thus repeated until the cutting is completed. Simple contrivances are 
adopted for so holding the diamond, with its stem, that it may be ground at 
any angle required. 
The forms produced by grinding the diamond are the brilliant and the 
rosette or rose (pl. 32, jigs. 85, 87, 90, 84, and 91). The object of giving 
so many facets to the diamond is to increase its sparkling, the light being 
reflected in all directions from the great number of variously inclined faces. 
The groups which succeed the metalloids, as the native metals, the 
tellurids, antimonids, arsenids, sulphurets, and selenids, are all valuable on 
account of the metals to be obtained from them. The native metals 
are mixtures of various metallic substances. Native gold, for instance, 
always contains silver; native platinum almost always includes with it 
iridium, osmium, ‘rhodium, palladium, ruthenium, iron, and manganese. The 
other groups contain ores of metals. A metal is found in chemical combi- 
nation with a metalloid, as sulphur, selenium, tellurium, arsenic, and 
antimony. Economical purposes, therefore, require that the native metals, 
which are nearly all of the precious class, be separated from these various 
mixtures, and that all the components be preserved. In the other classes, on 
the other hand, the metal is generally preserved at the expense of the 
metalloid, which is mostly lost. All of these, excepting silver, belong to the 
baser metals. The separation of the noble metals is generally effected by 
solution and precipitation of the individual species from the solution. The 
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