When one thinks of coral as a color, we envision an orangey medium red color and this color can exist naturally, but there are many natural variations of this color.
Generally you will find two types of jewelry coral - Sponge and Bamboo
Sponge Coral is formed by tiny sea creatures (polyps) that grow and die over generations and leaves a porous exoskeleton. These holes left by the polyps makes the coral look like a sponge and hence the name.
Bamboo Coral is a branchlike coral and has an appearance similar to bamboo. Natural Bamboo coral is grayish or inconsistent in color. This coral is generally dyed pink red or red for jewelry purposes.