SAPPHIRE.                                233
	fine sapphire on which is engraved the emperor Per-tinax. It is said there is in the Petersburg Museum a sapphire whose colour varies from whitish to clear blue, and on it is engraved a draped head ; so that the face is white and the folds blue, which produces a beautiful effect. I have a sapphire on which is the head of Vespasian in relievo ; it was found in the Appian Way. It was copied on a blue quartz by the gifted Odelli, a skilful Roman engraver, whose work, in my judgment, succeeded excellently ; but the artist assured me that the quartz cuts very badly on account of the want of closeness in the material. The same artist engraved for me two large blue corundums, and on them he succeeded in giving his work a beautiful finish, because, being harder, they offered more resist­ance to the tool.