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keep his Countenance when he ſees the Diamond reduced to a Species of Allum; and the Emerald of Borax, Foſſils Arrang'd, p. 137, 138.
What can be aſcertained in general is this:
The Maſs of conſtituent Matter in them all, is a pellucid cryſtalline Subſtance, which is in different Kinds of different Degrees of Hardneſs, from that of the Diamond to that of the mereſt ſhattery Cryſtal. This cryſtalline Matter, had it concreted in perfect Purity, had been colourleſs alike in all: and the various Species had been diſtinguiſhable only by their different Degrees of Hardneſs: but as this Matter, in the time of its Coaleſcence, aſſumed into it any Particles of a proper degree of Gravity and Fineneſs, which happened to float in its Way, it became by that Means different not only in Colour, nay, and in Degree of Colour, according to the Nature and Quantity of the Particles it took up into itſelf; but from their different Nature was alſo altered in what alone could have been its determinate Characteriſtics, its Hardneſs and ſpecific Gravity. Many Reaſons may be alledged why the Particles thus aſſumed into the cryſtalline Nodules at the Time of their Formation, muſt have been principally of the metalline Kind; and: we find, in effect, that they were ſo. The various Colours of the Gems have their Riſe from theſe Admixtures; and, according to what I have before obſerved as to the colouring of Spars by the ſame Means, when the metalline Matter thus mixed with the cryſtalline was Lead, the Stone became a Topaz, or, as the Antients called it, a Chryſolite: for it is very evident, that what they called the Topaz, we now call the Chryſolite; and what they called the Chryſolite, we now, on the contrary, call the Topaz.
Our Topaz is a very elegant and very beautiful Gem, of which the Jewellers have two Kinds, the Oriental and Occidental; the Oriental are of a fine pale yellow like the Jonquil Flower. They are of very great Splendour, and equal the Ruby in Hardneſs. Theſe are brought from Arabia, and many Parts of the Eaſt Indies. The Occidental are often very beautiful; but are diſtinguiſhed from the Oriental by their Softneſs, for they are no harder than common Cryſtal: and by a foxy redneſs with the yellow. We have them from Sileſia and Bohemia.
The Topaz of the Antients, now called the Chryſolite, differs from theſe in Colour, for it has always an Admixture of green with the yellow; probably from Particles of Copper diſſolved in an Acid, and taken up with thoſe of the Lead into the Matter of the Gem, at the Time of its original Concretion.
As theſe Gems have their Colours from this accidental Admixture of extraneous Particles, they may alſo be diveſted of them by Fire; without any Injury to their Texture: and the Oriental Topaz thus rendered colourleſs, is, like ſome other Gems to be hereafter deſcribed, ſometimes made to counterfeit a Diamond.
