Before washing his hands, Perseus delicately sets the serpent-crowned head onto a bed of leaves so that it won't be injured. But, on contact, the supple seaweed stems absorb the monsters power and harden. Henceforth coral - called gorgonion in Greek- possesses the property of becoming mineral if exposed to air. In water, its branches are flexible, but as soon as it emerges it turns to stone, saxum. In Ovid it is the Gorgon's contagious touch that petrifies, but in other versions it is her gaze.
Kristeva/ The Severed Head p29
The origin of this scene is a seldom-illustrated passage from Book IV of Ovid's Metamorphoses, describing how Perseus created coral. Having slain the Medusa, one of the three Gorgons, whose gaze turned all who saw her to stone, Perseus witnessed the birth of the winged white horse Pegasus, who sprang from her blood. Mounted on Pegasus with the Medusa's head in a sack, Perseus flew over an island where he saw Andromeda chained to a cliff and threatened by a sea-monster. Perseus slew the monster, freed Andromeda and began washing the creature's blood from his hands, placing the Medusa's head on some seaweed at the water's edge as he did so. The Medusa's blood turned the seaweed into a red stone - coral. Delighted with the effect, the nymphs soaked other algae in the blood, to make more.