A plain disc of travertine held off the wall, with the light source hidden entirely behind it. You see no lamp, no diffuser and no edge detail, just a stone circle with a ring of warm light escaping all the way round it.
The face stays unlit, which is unusual and is the whole effect. Most wall lights ask you to look at a bright thing. This one asks you to look at a piece of stone that happens to be surrounded by light, so the surface reads as texture rather than as glare, even at night.
Where it earns its place
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Living rooms. Bright enough to lift a corner, never bright enough to look at. It is the fitting you leave on all evening.
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Above a sideboard or a fireplace. Reads as a wall piece first and a light second, which is what that position wants.
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Long hallways. Three of the 20 cm at even spacing, or one 30 cm as a full stop at the end.
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Three sizes. 20 cm to punctuate, 25 cm as the general answer, 30 cm when it is the only thing on the wall.
Real stone, so no two are the same
A flat disc is the most honest cut there is: one full face of the block, nothing turned away from you, nothing hidden behind a wooden collar. Whatever seams and pores that slab carries are what you get, and the halo behind it grazes them from the edge, so the surface looks different again at night.
Fitting it
Hard wired and a job for a qualified electrician. At 4.5 cm deep it sits almost flat, so it works on a wall where a projecting fitting would be caught by a door or a chair. Decide the size against the wall, not against the room: 30 cm looks generous alone and crowded in a run of three. In a hallway, centre it at eye height, around 1.6 m.