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

204 GEMS.Between the emerald corundum and the Westernemerald of fine quality there is the same difference asbetween the perfect red corundum and the perfectspinel.The Western emerald is a silicate of alumina, almostidentical in its composition with the "beryl or aquamarine.It is the least hard amongst gems,breaks easily.and thereforeIt crystallizes in truncated hexagonal prisms attwo extremities ; its crystalsthelie between the compositerocks and argillaceous schists, and also in theaccidental cavities which occur in granite blocks.Sometimes it is found united with crystals of quartz,mica, and felspar. It is, moreover, procured from sulphurettediron, in carbonate of lime, and in sulphurettedlime.This emerald has a vitreous and seldom aresinouslight. It is soft when just taken from the mine, buthardens in the open air.Its specific gravity is between 2'73 and 2-76. Itshardness between 7 '05 and 8. It yields a whitepowder. It becomes electric by friction. It meltsunder the action of the blow-pipe, forming a limpidvesiculated glass of the consistency of borax. Whencalcined and still hot, if thrown into water, it breaksinto fragments of various colours.It gives, on analysis,