Corallium rubrum belongs to the octocorals, whose polyps carry eight feathery tentacles, rather than the hexacorals that build tropical reefs. Its skeleton is a solid axis of high-magnesium calcite coloured red by carotenoid pigments, which is what makes it workable as jewellery rather than the porous aragonite of a reef coral.
It lives without zooxanthellae, the symbiotic algae that power reef-building corals, and is therefore not constrained to sunlit water. Colonies occupy shaded overhangs and caves from shallow depths down to several hundred metres, feeding on plankton and particulate matter carried by currents.
Growth is extremely slow. Measured rates for Mediterranean populations are commonly a fraction of a millimetre in branch extension per year and a few percent in diameter, meaning a colony of harvestable size represents decades. Centuries of harvesting have shifted populations towards smaller, younger, shallower colonies, and the species is assessed as Endangered. The frequent error is treating it as interchangeable with tropical reef coral in both biology and recovery time.