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

INTEODUCTION. 5The natural characteristics of gems are :1st. The properties derived from the nature of thesubstance itself; namely, hardness and specific gravity.2nd. Those phenomena which are produced moreimmediately by some external influence as; light,and heat.HARDNESS.electricity,In gems, hardness denotes not only tenacity, but alsothe greater or less resistance which they oppose tobeing cut, scratched, or polished by others ; thus,while the diamond from its small tenacity is easilybroken, and does not give out sparks under the actionof the steel, it is the hardest amongst gems, becauseit cuts all other bodies, without being itself markedby them.The hardness of gems, considered as the attractionof cohesion that is, in the power they have of resistingthe blows they receive to break and divide themmechanically differs considerably in the same species,according to the direction of the cutting, the surfaceof the stone on which the trial is made, and its moreor less perfect quality.Hardness, and not cohesion, is the basis on whichgeneral experience rests for determiningto whatspecies a given gem belongs. For such a purpose,practitioners adopt the file but;the mineralogist rubsthe stone which is to be tried against another, beginningwith the hardest gem, which is the diamond,