Carnelian is a clear red translucent chalcedony, and is usually of a
gray or grayish-red color. Several weeks of exposure to the sun’s rays
and subsequent heating in earthen pots enhances and deepens the color.
The brownish-red or dark-brown carnelian is called sardoine or sard;
the blood-red to pink varieties, with an upper layer of white onyx, are
called carnelian onyx, and the stones with a brown or sard base and a
white top are called sard-onyx.
Carnelians are sometimes of a yellowish-brown or yellow color, but red
to brown are the principal colors.
The secret of coloring agates was discovered in the early part of this
century, and about the same time agates became scarce in Oberstein,
while large finds were made in Brazil and Uruguay, especially of agates
with red layers. This variety comes chiefly from Brazil.
Besides Uruguay and Brazil, carnelian is found in Arabia and India. The
most beautiful specimens of intaglios are engraved on sardoine, and
some of the finest cameos extant are of sard and carnelian onyx.