Carnelian (or cornelian) is the translucent red variety of chalcedony which in its turn is a form of cryptocrystalline quartz. It's been known and used since antiquity when it was sourced from the surfaces of the Egyptian and Arabian deserts. The highliest prized carnelian has a true blood-red color.
Carnelian can be created by heating nearly colorless chalcedony, turning it red. Today, naturally colored carnelian is rare, it is much more common to find artificially colored (dyed) material. It is usually carved into intaglios or cameos or fashioned as beads or cabochons.