The one that does not look like seaweed is pink. It is hard. It is calcified, covered in a layer of calcium carbonate, like a coral or a shell. You can tap it with your fingernail and hear a click. You can try to tear it and fail. It is more rock than plant, but it is alive, growing slowly, adding layer after layer of pink stone. It is the alga of the hard surface, the one that blurs the line between plant and mineral. What makes it special is the hardness. The coralline algae deposits calcium carbonate in its cell walls, creating a rigid, protective skeleton. This skeleton protects it from grazing animals, snails and sea urchins find it too hard to eat. It also helps it withstand the pounding of the waves. The coralline algae is the armour-plated tank of the seaweed world, the one that laughs at the teeth of snails. The coralline algae comes in two forms. The crustose form grows as a flat, pink crust on rocks and shells, spreading slowly across the surface like a layer of paint. The articulated form grows as branching, tufted, jointed fronds, like tiny pink trees. Both forms are hard and brittle, snapping rather than bending. Under a hand lens, the articulated form is beautiful, each segment a tiny pink cylinder, the joints flexible like a spine. Biologically, the coralline algae is a red alga. It reproduces by releasing spores from specialised structures on its fronds. The spores are released into the water, carried by the currents, and settle on nearby rocks to grow into new plants. It grows slowly, a crustose patch may take years to cover a handspan of rock. To find coralline algae is to find a flash of pink on the dark rock. The crusts are smooth and hard, like a layer of pink paint. The branching tufts are brittle and delicate, snapping at a touch. It is the rock that is alive, the seaweed that is stone, the pink that grows in the dark, the one that proves that the boundary between life and stone is not as sharp as we think.