Carnelian and onyx: two names that get misused constantly

Carnelian is simply an orange to reddish brown variety of chalcedony, colored by iron oxide, and it is genuinely natural in most cases since the color forms readily without treatment. Onyx is more complicated, true onyx is a banded chalcedony with straight, parallel black and white bands, but a large share of the black banded material sold commercially as onyx is actually ordinary gray agate that has been dyed black, a very old and common lapidary practice. Neither fact makes either material less legitimate to collect or use, but it is worth knowing that the name alone does not guarantee natural color, especially on uniformly jet black banded pieces.