Birjusa (Turquoise)

Oriental Turquoise was formerly esteemed, and very deservedly, the most beautiful of opaque gems, but the substitution of fossil bones or teeth, without their having; been generally distinguished, and numerous imitations, have lessened its value in Europe : still, however, it is a principal article of commerce^* among the Persians, Russians and Turks. Mineralogists have for a long time considered both the occidental and oriental Turquoise, or the Nouvelle and Vieille Roche, as fossil teeth and bones, penetrated by Calx or Carbonate of Copper; and, notwithstanding Reaumur and others have pointed out differences between them, most modern Mineralogists have either wholly excluded Torquoise from their systems, or spoken very slightly about it. It appears that Lowitz was acquainted with the true oriental Turquoise, and found in it a good deal of Argill, with a little Copper andiron; and that Bruckman considering this, and the situation of the substance from Korosan (or Chorossan) was induced to think it a distinct mineral: however, the prevalence of coloured fossil bone among collectors, has misled others; and Bouillon Lagrange having analysed this, determined later Mineralogists in favour of the opinion, that all the Turquoise was either fossil bone or the enamel of the teeth of some large animal, coloured by Phosphate of Iron, and not Copper as had been previously supposed. Mr. Konig informs me that John has given the analysis of Turquoise, from Korosan, which confirms Bruckman's opinion; he has himself placed it in the Museum collection, and Synopsis among the hydrates of Alumine.

Jewellers and Turkish merchants distinguish between the fossil bone and the true Turquoise ; the latter being known by the epithet Vieille Roche or called Birjusa: it occurs in veins and tuberose masses, in a greyish black stone, resembling the silicious schistus, in which the Hydrargillite of Barnstaple is found^†. There are probably several mines that are worked for it, all situated in Korosan, in the north east part of Persia, Bouillon Lagrange^‡ mentions two: one is three days journey from Meched, called the Old Rock, which is only dug on account of the King of Persia; the other, five days journey from the same place, produces Turquoises of a pale colour, that are sold at a very low price. The specimen I have represented on tab. 93. is from a mine at Nischassur, in Korosan, although of a poor colour, it was purchased at a high price by Mr. Heuland, to whom I am indebted for the Russian name, and who assures me that the genuine oriental Turquoise or Birjusa, is found in no other country; but that stained fossil bone is frequently understood by the name Turquoise among the Jewellers of the continent, in opposition to that of Vieille Roche. The figures upon tab. 92. are from several specimens presented to me by Mrs. Liston, whose kindness in collecting illustrative minerals from the cast, I must be allowed to express my gratitude for. They are from a new mine, in which, when the works are more advanced, it is expected that fine stones will be found, many of those already obtained being of a good colour and texture, but defective in thickness. Sir Charles Giesecké upon examining some of these and others communicated by Mr. Heuland, has found them to consist principally of Alumina, with a little water, coloured by Copper, thus further confirming Lowitz’s analysis, and Bruckman’s opinion. I have given for illustration, a figure of the bone Turquoise, from a fine specimen belonging to Mr. Walker, of Arno’s Grove; its laminated structure will distinguish it from the mineral Turquoise; I suspect it would not retain its colour so well as the genuine.