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they engrave Seals. Its colour is red,
But if this be the Caſe in foſſile Subſtances in general, it is much more particularly ſo in this Claſs of them, the Gems; the Differences of which are owing to the Diſtribution of a certain kind of Particles in their Maſſes; which are ſo very uncertain, both in Quantity and Manner of placing, and in their various Effects upon the Maſs, that ſcarce any thing abſolute is to be determined from them.
The Gems are naturally angular, as are the Cryſtals: but like them, from various Accidents in their Formation, they are found ſometimes in rude or ſhapeleſs Maſſes; and when angular, they have ſtill all that Variation of Figure which we ſee take place in Cryſtal and Spar; from the different Diſturbances of their Cryſtalization. In all theſe Caſes a various Number of Angles may be occaſioned, as we ſee in Salts, from the Accidents of their Concretion. In theſe, as well as in thoſe, we have the ſame Kind in different Figures; and as we can cryſtalize them under the Eye, we can determine the Cauſes of thoſe Alterations. The round, or pebble Gems, ſeem not to have been original in that Form, but worn to it by rolling about in a Fluid.
The Hardneſs and the Luſtre of the Gems, muſt diſtinguiſh them from all other Stones; for if we conſidered their Form, as their eſſential Character, many Cryſtals would aſſume the Name: and Cronſtedt has well determined, that a certain Spar he had ſeen in Figure of the moſt regular Diamond, muſt then be called, a Diamond.
No peculiar Conſtruction, no Form of conſtituent Parts is viſible in the Gems: they appear as Maſſes of uniform Nature; and they break irregularly and indeterminately; yet there is in all a really plated Structure. The Lapidaries find this in ſome, and can ſplit them; the Burning Glaſs diſcovers it in the reſt; and when turned to it in a right Direction, tears them to pieces: they ſplit into the thinneſt Plates that can be conceived, and ſeem to have been compoſed in the Manner of the Talcs, only more compact. 'Tis pity this Character is not more obvious: for it affords a real diſtinctive Mark between the Gems, and all other Stones: Cryſtals, which ſeem to come neareſt to them, have it not.
Their Colours are leſs eſſential, for they can in moſt be driven away by Fire; and Nature ſometimes gives the Gem without them; they are evidently owing to the Metals; for we can by means of Metals, give the ſame to Glaſs; our artificial Gem.
The Salt Syſtem of Linnæus appears here almoſt ludicrous. To a truly philoſophic Eye, the Difference of Eſtimation and Price are nothing; but the common Reader will hardly
