Seam four

Travertine forms in water. Mineral-rich springs deposit calcium carbonate in layers, and gas escapes upward through those layers as they set, leaving the open channels that give the stone its face. It is, geologically, a record of bubbles.

Almost all commercial travertine is sold filled. Resin or cement is pushed into the pores, the slab is ground flat, and what you get is a beige surface with a pattern printed into it. It is easier to clean and it is much easier to sell.

Seam four at Tivoli is a metre-thick band about nine metres down, and it is unusually open — more pore than most quarrymen want. We buy it unfilled and we buy it by the block, which means we take what the seam gives us and design afterwards.

The objection is practical: unfilled travertine collects dust and marks with oil. This is true. It also means that in ten years the object will be visibly older, in the way a leather bag is visibly older, rather than in the way a laminate worktop is visibly older.

We spent two days at the face with Kai Brandt, who cuts for us, watching a block come out. Everything below the twelfth metre is denser and quieter and better behaved. We keep buying from four.