How do I remove limescale from a marble shower without ruining the marble?

Why the usual answer works

Limescale is calcium carbonate deposited from hard water. Acids react with carbonates to give soluble salts and carbon dioxide, so the deposit dissolves and rinses away rather than having to be scrubbed off. This is why every descaler sold is acidic, and why the advice to use vinegar or a limescale remover is not wrong about the chemistry.

Usually recommended: Acetic acid (vinegar), Citric acid.

Why it must not be used here

Marble is also calcium carbonate. The acid cannot distinguish between carbonate deposited by hard water and carbonate that is the stone itself, so it dissolves both. The result on the stone is etching: a dull, slightly rough patch where the polished surface has been chemically removed. It is not a stain and there is nothing left on the surface to clean off.

This damage cannot be undone by cleaning. There is nothing left on the surface to remove.

More about polished marble and limescale.

What to do instead

Take the mechanical route rather than the chemical one. The stone trade's stated treatment for surface accumulation of hard water — what it calls water spots and rings — is to buff with dry 0000 steel wool, which is fine enough to lift the mineral film without cutting the stone, and involves no acid at all. Work on the deposit only, keep it dry, and stop as soon as the film has gone. Prevention does more than any treatment here: a squeegee after each use removes the water before it can evaporate and leave its minerals behind, which is the moment the deposit is actually created. If the surface is already dull in patches rather than filmed, that is etching rather than scale, no cleaning method will touch it, and re-polishing is a job for a stone restorer.

• Mild abrasion — Removing a deposit with particles softer than the surface beneath it. Safe or ruinous depending entirely on relative hardness.
• Neutral cleaner — A near-neutral surfactant cleaner. The recommended answer for natural stone of every kind, and the least interesting product in the cupboard for good reasons.
• Rinsing and drying — The step people skip, and the one that decides whether cleaning protected the surface or damaged it. Residue left behind is exposure that continues after you stop.

This is a case where a specialist is a reasonable answer. Restoring a surface is a different trade from cleaning it, and continuing to try cleaning products on damage generally makes the eventual restoration harder.

The chemistry behind both halves

• Etching versus staining — A stain sits on a surface and a mark from etching is a hole in it. They look similar, they need opposite responses, and cleaning harder makes one of them worse.
• Water hardness — Dissolved calcium and magnesium picked up from the ground. It decides how much detergent you need, what your kettle looks like inside, and whether surfaces dry clean.
• Material compatibility — The reason a useful answer is about the surface rather than about the product. The same agent is the right answer and the wrong answer depending on what it lands on.
• Mechanical action and abrasion — Physical force does a large share of all cleaning, and abrasive hardness relative to the surface decides whether it cleans or destroys.