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

182 G-EM8.gave it we cannot know where they procured it, butfine pieces, both rough and worked, are often foundamongst the remains of Italian monuments.It appears 'that the plasma was only used ornamentallyduring the time of the Lower Roman Empire,as the subjects of the ancient intagli and the qualityof the execution bear witness. Contrary to my opinion,to which for the above mentioned reasons I firmlyadhere, Emmanuel assertsthat even the Greeks calledthis stone plasma, because its name was derived fromthe word TrA-aayxa, image; and Barbot, misled by thesame ambiguity, says that the plasma comes only fromIndia, and that in ancient times it was probablythence brought to wealthy Carthage, and from thereto Rome.LXXXII.PORPHYRY.ALTHOUGH porphyryis not a gem, we think it rightto make some mention of it, as we did of marble, asmuch for the purpose of following the example of thosewho have written on the subject of gems as becausemany works of art have been executed in that material -Porphyry is composed of a felspar rock called, scientifically,mixed l&ptinite, with crystals of ortosa andalbite. The different kinds are distinguished by theirground colour, in red, green, white, brown, or black.Their specific gravity varies, according to theiraggregated substance, from 2'69 to I'll.