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color is not such as desired, they are heated until it is brought 
to the desired tint. The greater part of them however in their 
natural state, are of a dirty greyish color, with bands of white 
of greater or less thickness running through them. By long 
habit the workman acquires the art of being able to distin- 
guish those bands which will absorb color, and the pieces 
are cut accordingly with diamond dust. Prepared in the rough 
they all have the same color. The colors most sought for 
in commerce are various shades of red, green, yellow and 
blue. The red color is given by allowing the stone to 
remain several days in an acid solution of iron, the time de- 
pending on the depth of color to be given, and the shade on 
the absorbent power of the bands. It is then taken out and 
placed in an oven, where it is kept for some time at a 
temperature of between 100 and 200° Cent., until no further 
change takes place. The stone is then red and is cut, accord- 
ing to the depth of the color in the different bands, with 
a diamond saw, ground on large wheels of sandstone, before 
which the cutter les on his breast, and polished on a rapidly 
rotating wooden wheel with tin. 
The black color is given by boiling first in honey and then 
in sulphuric acid. The bands which absorb the honey, be- 
come black from its decomposition. Those which do not, 
remain white. This is the onyx of commerce. When there 
are no bands of white the stone is called jet. 
The yellow color is produced by Chromic acid. To be cer- 
tain of a good tint, the stones are first boiled in Hydrochloric 
acid to clean them and dissolve out anything that may 
be soluble. They are then placed in Chromic acid and left 
for several days. It then shows yellow where the acid has 
penetrated. 
To give the beautiful green color which is so much sought 
for at present, the. stone which has been steeped in Chromic 
acid, is covered with carbonate of ammonia and heated. 
The blue color is given by placing the pieces to be treated 
first in a solution of Ferrocyanide of potassium, then in a salt 
of iron. All of the artificially prepared stones receive their 
definite form and polish after they have been colored. | 
