262	NATURAL HISTORY OF GEMS.
	the portico to Constantine's Baptistery at Rome, and in the mosque of Sta. Sophia. It was a fasionable material for the lower parts of the later imperial busts, having the head alone in white marble or bronze, its colour aptly reproducing the sovereign purple (a dark crimson dye). Those ages have left to us certain works in Porphyry, the execution of which remains a mystery ; perhaps the most wonderful things the Roman sculptor ever produced, con­sidering the elaborateness and high relief of the decorations, and the invincibility of the material employed. The mag­nitude of the original blocks is likewise a subject for admiration. Of these monuments the principal are the Sarcophagus of the Empress Helena, in a single block, 194 palms high by 12 long (13x8 ft.) of the finest texture and deepest colour. It is adorned on the two fronts with a military procession, the " Triumph of Constantine," com­prising many figures on horse and foot. Busts of Helena and Constantine stand out in the highest relief, as medal­lions above. Placed at first in the Mausoleum of Helena (now the Torre Pignattara), it was removed thence to the Lateran by Anastatius IV., in 1154, to serve for his own tomb; and was set close to the Porta Santa until 1600, when it fell to pieces on being removed in the course of the alterations of that Basilica. The fragments remained in the cloister until restored and placed in the Vatican Gallery by Pius VII. The restoration and repolishing of this monument occupied many hands continuously during seven years. The second, the Sarcophagus of Constantia, daughter of Constantine, is much less elaborate in its decorations, besides being of very inferior workmanship to the pre­ceding. Cut like that out of a single block, 11 palms long by 8 high (7-1/2 x 5-1/2 ft.), the two fronts present groups, prettily designed, of Cupids engaged in the vintage : below