Plunge basins carved from the stone of the place

Five stones, five countries

Pink-grey granite in the Seychelles, where the Inner Islands are the only granitic ocean islands on earth and the boulder is already on your plot. Dense black volcanic basalt in Mauritius and Kenya. Pale porous coral rag on Zanzibar, which has been used in that architecture for centuries. Creamy alabaster in Egypt, where the quarrying tradition is older than almost anything else in this business. Cast indigo terrazzo in Lagos, which is not quarried but is unmistakably of the place.

Each behaves differently in water and each is honed differently. What they share is mass, chemical stability and complete indifference to salt air, which is close to ideal for African coasts.

Honed inside, split outside

The block is split, the inner bowl cut and worked to a finish smooth enough to sit against for ten minutes, and the outer faces left raw. That contrast is the whole design: from outside it reads as the boulder it came from, inside it is precise. Polishing the outside would make it look like a worktop and lose the point.

The bowl gets a profile rather than a bucket shape, so an adult sits with shoulders under water and shins comfortable. Getting that right takes longer than the rough cutting, which is why we would rather you sat in one before ordering.

The insulation nobody sees

Stone has enormous thermal mass and is not an insulator. A basin with cold water in it and nothing behind it bleeds heat into the ground and the air continuously, and the chiller pays for that every hour.

So behind the stone there is rigid insulation and a waterproof backing doing the thermal and structural work. The stone is the finish and the mass. Done that way the mass becomes an advantage: the basin holds temperature beautifully overnight under a lid, and run properly it is cheaper than a clad tub rather than dearer.

Weight, and the conversation it forces

A carved basin is measured in tonnes before any water goes into it. On a ground-level terrace over rock that is a non-issue. On a suspended slab, a balcony, a tower terrace or a timber deck it is a structural question with a real answer, and we get an engineer's opinion before quoting rather than after.

This is one of very few things we refuse outright: a stone basin on a structure nobody has assessed. Where the structure will not take it, the honest alternative is an insulated clad tub — lighter, cheaper, and it works extremely well. It simply is not carved from the island.

In short

• Granite, basalt, coral rag, alabaster or terrazzo, chosen by country
• Honed bowl, split outer faces, ergonomic profile rather than a bucket
• Rigid insulation and a waterproof backing behind the stone
• Engineer's opinion on any suspended structure before we quote