Seashell

Seashells sit on sand in the Ocean, both on the dry beach and out under the water, and the placed tile picks one of three looks at random when it appears. What separates them from most world generation decor is that they grow back, so the Ocean is a renewable supply rather than a one time harvest. Starfish share the tile, the spawn rules and the internal handling, so anything true of one is true of the other.

Crowding is the main brake. Once a few shells or Coral sit within a short radius the rate drops, and past a small count in an even tighter radius nothing new appears at all. Spread the sand out rather than packing it. Weather pushes the rate the other way: rain helps, wind helps, and both at full strength is the best window you get. Open space above the sand multiplies it too, counting anything up to fourteen tiles overhead, and water, background walls and inactive blocks all count as open.

That headroom rule gives you the design. Stack sand on platforms fourteen tiles apart inside the Ocean and every layer produces at close to full rate. It keeps working above the surface, right up into Space, so height is free real estate. Backing the layers with walls suppresses the plant growth that would otherwise share the sand without hurting shell rates at all.

The same roll that hands you common shells occasionally produces a Junonia Shell, and that one sells for enough to fund an early run on its own. Tulip Shell and Lightning Whelk Shell land in between. Do not stockpile the fancy ones for crafting, though. Recipes that accept any seashell or starfish, including Reef Block and Shell Pile, only take the plain shell and the starfish. The rarer three are money and decoration, nothing else.