Gems Notes and Extracts Augusto Castellani, Mrs. John Brogden 1871

INTBODUCTION. 7represents the specific gravity of the gemwhich hasbeen put to the proof.It is often very useful to know the specific gravityof gems, as a means of determining their quality, andto prevent the danger of their being changed for others,similar in appearance but very different in value ; as,for instance, the diamond and the jargoon.In the case of cut and polishedgemsit is thereforeof great use in ascertaining the class to which theybelong, without the possibility of injuring them bythe otherwise necessary operation of filing.The comparison of the specific weight of gems was,according to Emmanuel, known to, and practised by,the ancients, and certainly in the Indies, for manycenturies, in order to ascertain the quality of a preciousstone.BRIGHTNESS, COLOUR, AND TRANSPARENCY.Gems acquire more or lessbrightness, lively, variedcolouring, and transparency, in proportionto theirsingle or double refraction, and to the polarization oflight.The brightness of gems may be distinguished asfollows :I. Adamantine brightness; that which gives outthe splendour peculiar to the diamond.II. Vitreous brightness; that which resembles glass.III. Eesinous brightness; that which shines as ifthe surface had been rubbed with oil.