Natural red coral, often found in old jewellery pieces, is undergoing a modern renaissance as both jewellers and customers are appreciating its natural beauty and lustre again. Unfortunately, what is sold as red coral is often a man-made imitation of the real thing using plastic, glass, dyed bone and shells. Red coral is seldom found in reefs, but is the calcified remains of a marine animal that form branchlike structures in colonies.
Examine the piece of coral under high magnification. Real red coral has a distinctive, visible texture, similar to wood grain, that you can see under magnification. The most common imitators, such as hard plastic, glass or even bone and shells, do not have that texture.
Put the coral into a glass of milk. Real red coral will cause the milk to take on a tinge of red or pink. Neither plastic nor glass alternatives will cause this change. Dyed bone or shell can discolour the milk. If the milk colour does not change, you do not have real coral. If it does, you might have real coral.
Look for bubbles or mould lines in the piece. Glass facsimiles often have tiny bubbles embedded in them, while you can see tiny mould lines in some of the plastic imitations. Real red coral never has bubbles in it.
Put the piece into a bowl of heavily diluted vinegar. Real red coral will form bubbles as the calcium carbonate in it evanesces in the presence of the vinegar's acidic environment. Again, though, the best imitations made of dyed bone or shell will do the same. If the piece does not create bubbles, it is not real coral. If it does, it might be coral.
Look at the combination of the tests. While each test does not rule out all imitators, a piece that passes all tests is very likely to be real red coral.
There are other tests a jeweller can perform that will definitively establish whether the piece is real red coral or not. Some of these tests can seriously damage the jewellery if attempted by an amateur. If your jewellery passes all the tests listed here, it is worth your while to confirm the authenticity with a qualified jeweller or gemologist.
Remove the jewel from its setting if you safely can before testing. The tests can be done while it is in the setting, but that can result in some false results. If you are working with a very old piece of jewellery, it is often best to leave it as is rather than chance damaging the setting.