Antipatharian corals lay down a proteinaceous skeleton in concentric layers around a central axis, adding material at rates that can be measured in tens of micrometres per year. Radiocarbon measurements on colonies from deep Hawaiian waters have produced ages exceeding four thousand years for the oldest specimens examined, and comparable work on the gold coral Kulamanamana has returned similar figures.
That longevity makes the skeleton an archive. Because the coral feeds on sinking particulate matter, the layers carry an isotopic record of what was in the surface water above at the time they formed, giving a proxy series that extends centuries or longer.
Two corrections are worth making. The living animal is not black; polyps are typically white, orange or red, and it is the skeleton beneath that is dark. And most black coral is not shallow reef life at all but deep, cold and slow, which is why removal for jewellery is effectively permanent within any human timescale. Black corals are listed on CITES Appendix II, restricting international trade.