Two beaches within a kilometre of each other, on the same stretch of coast, facing the same way. One is a pleasure to spend a day on and the other is genuinely uncomfortable. That difference is not luck and it is largely predictable.
Where sand comes from determines what it is
Sand is whatever the local rock has been broken down into, plus whatever the sea has brought in, and those two sources produce very different material.
Beaches near granite or similar hard rock tend toward coarse, pale, quartz-heavy sand that drains fast and stays relatively cool.
Beaches near volcanic rock produce dark sand, sometimes almost black, which absorbs heat and can become genuinely too hot to walk on barefoot by midday. This surprises people arriving at a beautiful black beach and discovering they cannot cross it.
Beaches formed from broken shell and coral are the pale fine sand people imagine when they think of a beach. They stay cooler, they are soft underfoot, and they are largely a warm water phenomenon because the organisms that produce them live there.
And beaches near soft sedimentary rock or river mouths get finer, siltier material that compacts hard and can be sticky rather than loose.
Grain size is what you actually feel
The single most consequential property and the one nobody thinks about.
Fine sand packs firmly when wet, which makes it excellent to walk on and comfortable to lie on. It also gets everywhere, sticks to everything, and travels home in your bag for months.
Coarse sand is loose and tiring to walk on, particularly above the waterline where it is dry. It is much easier to brush off, and it does not compact into a firm surface at the water's edge.
Shingle is a different proposition entirely. Uncomfortable to lie on without a mat, noisy in a way that is either wonderful or intolerable depending on the person, and hard work to walk on. It also drains instantly, which means it dries fast and does not get muddy.
The slope tells you about the waves
A relationship worth knowing because it predicts both swimming and comfort.
Steep beaches are generally made of coarse material and are associated with more energetic waves. The water gets deep quickly, waves break close to shore with force, and there is often a strong backwash dragging at your legs.
Shallow, flat beaches are generally finer material and gentler conditions. The water stays shallow a long way out, which is what makes them good for children and poor for anybody wanting to swim properly without a long wade.
You can read this from photographs. A beach where the waves break right at the shore in a single dumping line is steep. One where you can see multiple lines of white water spread over a distance is shallow.
What this means for choosing
If the priority is small children, a shallow fine-sand beach with a gentle slope is what you want and nothing else is a substitute.
If the priority is swimming, a steeper beach gets you to depth quickly, at the cost of more energetic water at the entry point.
If the priority is walking, firm fine sand at the water's edge on a falling tide is by a distance the best surface, and dry coarse sand is the worst.
And if you are sensitive to heat underfoot, dark volcanic sand is worth avoiding in the middle of the day or requires shoes.
Beaches change and some of it is not natural
A thing I did not appreciate until I saw it happen at a place I knew.
Beaches are dynamic. Storms remove sand and deposit it offshore; gentler conditions bring it back. A beach can lose most of its sand over one winter and recover over a summer.
Some do not recover, because coastal defences, harbour walls or dredging elsewhere have interrupted the movement of sediment along the coast. Sand travels along a shoreline in a persistent direction, and anything that blocks that starves everything downstream.
Many popular beaches are now replenished artificially, with sand pumped or trucked in. Some of that imported sand is a different grain size from the original, which changes the character of the beach and is a common local complaint.
Which is worth knowing when a beach does not look like its photographs. It may not be the wrong beach; it may be a different year.
The practical checks before going
Satellite imagery shows the colour of the sand and the width at whatever tide the image was captured.
Visitor photographs show the texture and whether people are wearing shoes, which is a decent proxy for heat and for shingle.
Reviews mention shingle constantly, because people arriving expecting sand are reliably disappointed and reliably say so.
And the shape of the coastline tells you about shelter. A beach in a deep bay facing away from the prevailing wind is a different day out from an exposed straight shore, regardless of what the sand is made of.