Carnelian is a variety of agate quartz, the finest kind being of a cherry-red, with the semi-transparency of that fruit. This tint passes to an orange-yellow, more or less intense, and sometimes diffused throughout the mass, or occurring in irregular layers. The finest carnelians come from Japan, those found in Europe being either impure or of smaller size. Yet this stone is one of those most especially employed by the ancients for engraving. The Paris Cabinet of Medals possesses one, representing Bcllerophon mounted on Pegasus, apparently the work of Epitynchanus.

At the time of the Renaissance, the secret was discovered of discolouring the carnelian by artificial heat, and of giving it two distinct layers, one white on the surface, the other red, the object being to render it suitable for cameos. As already stated, the same process was employed to obtain the modelling in certain parts of the Florentine mosaics.