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74 PRECIOUS STONES
by which a precipitate of Prussian blue is formed in the pores. Another method is to use a solution of blue vitriol and ammonia. Green is obtained by impregnating the agate with salts of nickel or chromium; yellow, by the use of warm muriatic acid. There are many processes by which almost any tint may be obtained, but the exact methods are known to few. Most of this work is done in Oberstein and Idar, on the river Nahe.
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