Coral gave the ancient and medieval naturalists more trouble than almost any other material, because it does not behave like a plant, an animal or a stone, and they classified it in turn as all three.
Theophrastus discusses it among stones. Ovid supplies the aetiology that fixed the popular image: in the Metamorphoses, Perseus lays the head of Medusa on seaweed at the shore, the weed hardens at its touch, and the sea nymphs propagate the new substance. That story explains precisely the observation that puzzled everyone, which is that coral is soft and flexible in the water and hard in the air. Pliny reports the belief and adds his own observations.
The question was not settled until the eighteenth century, when the polyps were properly observed and coral was placed among the animals. Before that, the standing position for a long period was that it was a plant that turned to stone, and the intermediate category of zoophyte, plant-animal, was invented largely to accommodate it.
That history matters for reading the traditional attributions. When a lapidary treats coral alongside minerals, it is not being careless; the classification genuinely was open.
The modern position of the material is a conservation one. Precious coral grows extremely slowly, the shallow Mediterranean beds were worked out, and deep-water species are now under pressure from the same demand. International trade in several species is regulated, and a great deal of what is sold as coral is dyed bamboo coral, reconstituted powder or plastic.