Coral

From Softology's collection · B6-7,9,12/S5-8/C4

Grows outward like coral and keeps shifting once it reaches the walls.

How the rule works

Each cell has 26 neighbours: every cell that shares a face, an edge or a corner with it (the Moore neighbourhood).
An empty cell comes alive when 6, 7, 9 or 12 of its neighbours are alive.
A live cell stays alive when 5 to 8 of its neighbours are alive; otherwise it starts dying: for 2 generations it fades, is not counted as a neighbour and blocks births, then the cell is empty again.
With 4 states in all (alive, 2 dying, dead), this is a Generations rule.

It starts from a random block in the middle of the box, 50% full and half as wide as the box. Each run uses a new seed.

B lists the neighbour counts at which an empty cell is born, S those at which a live cell survives, and C the number of states. How to read a rule