A shell can only be added to at the edge
A mollusc builds its SHELL from the rim outwards and can never go back and alter what it made last year. Everything it has ever been is still there, in order, with the oldest part at the tip of the coil — which is the opposite of a bone, and the reason the RIDGES on a CLAM or COCKLE read like tree rings.
That constraint produces the shape. If the animal adds a little more each time in the same proportion, the only curve available is one that widens as it turns while keeping exactly the same shape — a logarithmic spiral. A baby NAUTILUS and an adult are the same object at different sizes. The SPIRAL is not a decision; it is what happens when something grows by addition and never remodels.
Almost every snail turns the same way
Hold a WHELK or a CONCH with the point upwards and the opening will, in the overwhelming majority of cases, be on the right. Shells are right-handed, and have been for most of the history of the group, for reasons buried in how the embryo first divides.
Left-handed ones do turn up, and they have a serious problem: their anatomy is mirrored, so they cannot physically mate with a normal snail. In 2016 a lefty garden snail found in London was briefly famous while scientists appealed for another. Two turned up. He was not interested in either.
Shells were money for longer than coins have been
The COWRIE — small, glossy, hard to counterfeit and conveniently uniform — served as currency across West Africa, South Asia, China and the Pacific for well over a thousand years. Chinese characters connected with money still carry the symbol for a cowrie shell, a fossil of the days before metal coins.
It worked because supply was limited and the shells were durable and countable. It stopped working in West Africa in the nineteenth century, when European traders shipped them in by the boatload from the Indian Ocean, flooded the market and destroyed the value — a very literal demonstration of what happens when you print too much money.
A pearl is a complaint
An OYSTER does not make a PEARL because it is pleased. When something irritating lodges inside — usually a parasite rather than the proverbial grain of SAND — the animal walls it off, coating it in layer after layer of the same smooth material it lines its shell with.
Enough layers and you have a pearl: a tiny intruder buried in shell. Natural ones are vanishingly rare, and almost every pearl sold today is cultured, meaning somebody inserted the irritant deliberately and waited. The shimmer comes from those layers being about the thickness of a wavelength of light, so the colours are light interfering with itself on the way out.
What is on the beach, and what the conch is really saying
Two basic designs wash up. Bivalves — the CLAM, COCKLE, MUSSEL, OYSTER, SCALLOP and RAZOR — are two matching halves on a HINGE. Gastropods — the WHELK, CONCH, COWRIE, NAUTILUS and the cone-shaped LIMPET — are a single coil. All of them are built from calcium carbonate, the same mineral as chalk and limestone, pulled straight out of seawater.
And the sea you hear in a CONCH is not the sea. It is the noise already around you — traffic, wind, your own blood — resonating in a hard curved cavity, which is why a mug works nearly as well. Best practice on the SHORE or COAST is to COLLECT only empty ones, check nothing has moved in, and leave the SMOOTH and occupied where they are in the SAND.