Red coral

This is the precious coral of Mediterranean jewellery, and in the water it looks nothing like the polished beads. Colonies are small, stiff, sparsely branched bushes, usually a hand's span or less, with a hard blood-red core wrapped in a thin red or orange living skin. The polyps are the giveaway: pure white, each with eight feathery tentacles, standing out like tiny snowflakes against the red when the colony is feeding.

It wants darkness more than depth. Around the Maltese islands look for it on cave ceilings, the roofs of deep overhangs and shaded vertical walls, usually hanging downward, from around 25 m to well past sport-diving depth. Shallow, accessible colonies are scarce because they were taken long ago, and Malta has a documented commercial coral fishery history - over a tonne of red coral was landed from Maltese waters in the mid-1980s. Offshore, surveys have since found it alive at astonishing depths, including records from well over a kilometre down in Maltese waters.

It grows a few millimetres a year at best and colonies live for decades, so the small bush in a Maltese cave roof is older than most of the divers looking at it.