It is possible to determine whether or not a necklace is real coral by dabbing it in an inconspicuous area with a cotton swab dipped in a bit of lemon juice. If it is real, small bubbles form on the surface of the coral.
Real coral is composed of calcium carbonate. The lemon juice effervesces and dissolves real coral. It is important to rinse the lemon juice off the necklace after testing it, because lemon juice damages the coral.
Determining if the coral has been dyed is done by rubbing the necklace in an inconspicuous area with a cotton swab dipped in acetone; the color comes off onto the swab if the coral is dyed.