Marble is the word listings reach for when they mean stone. Sometimes it is accurate. Often the material is granite, or quartzite, or a porcelain slab printed to look like Calacatta, and all four photograph identically. They do not behave identically, they do not cost the same, and the difference matters most in the room where people actually notice: the bathroom.
The four materials, plainly
Marble is metamorphosed limestone. It is calcium carbonate, which means it is soft enough to scratch and reactive to acid — lemon, wine, some cleaning products — which leaves a dull mark called an etch. It is also the most beautiful of the four, with the deepest veining and a slight translucency at the edge that nothing else has.
Granite is igneous, much harder, and effectively immune to household acids. It takes a high polish and holds it. The pattern is granular rather than veined, which reads as busier.
Quartzite is metamorphosed sandstone, and it is the one people mean when they say a stone looks like marble and behaves like granite. It has the veining and the hardness. Brazil is one of the largest sources in the world — Taj Mahal, Mont Blanc and Macaubas all come out of Brazilian quarries — which is why Brazilian houses use so much of it and why it turns up in trophy houses worldwide.
Porcelain slabs are manufactured. Large-format, thin, non-porous, printed with a stone pattern and effectively indestructible in a bathroom. Up close the pattern repeats and the edge gives it away.
“Quartzite is the reason Brazilian stone ended up in half the world's trophy houses. It has the veining and the hardness.”
Honed against polished
The same slab can be finished two ways and the choice changes the room more than the stone does.
Polished is the mirror finish. It brings out the colour and the depth of the veining, and it shows every water spot and every etch. In a bathroom in a hot climate it also shows every fingerprint.
Honed is ground flat and left matte. Quieter, softer, considerably less slippery underfoot when wet, and much more forgiving of marks — because there is no gloss for a dull spot to interrupt. Most well-detailed bathrooms use honed on the floor and polished, if anywhere, on a vertical surface.
Book-matching, and why it costs
When a block is cut into slabs, consecutive slabs are near-mirror images of each other. Open two of them like a book and the veining runs out from the joint symmetrically. That is book-matching, and it is the detail that separates a stone wall that looks designed from one that looks tiled.
It costs because it forces you to buy from a single block, cut in sequence, with the layout planned before anything is fabricated. There is no second chance: once the block is used up, that pattern does not exist anywhere else in the world.
How to tell what you are looking at
Look at an exposed edge. A stone slab is the same material all the way through; a porcelain slab has a printed face over a pale body, and the edge shows it unless it has been mitred.
Look for the repeat. Stone never repeats. Printed porcelain repeats every few square metres if you know to look.
And look at the drainage. A stone shower floor should fall to its outlet cleanly; standing water on stone is a detailing failure, not a material one.
Living with it
Marble in a bathroom is sealed, and the seal is renewed — how often depends on the product and the use, and any decent stone contractor will tell you the schedule for the specific material. Cleaning is with a pH-neutral product, not a bathroom spray with citric or acetic acid in it. That is the entire discipline, and it is easy once the household knows it.
Etches on honed marble can be polished out locally by a stone restorer. Etches on polished marble are more visible and more work. This is another argument for honed.
What to ask
Ask what the stone is, by name, and where it came from. A seller who knows the answer usually has the fabricator's paperwork, which also tells you when it was installed and by whom. Ask whether the vertical surfaces are book-matched. Ask when it was last sealed.
Then run a hand along the joint between two slabs. A tight, level, colour-matched joint is the mark of a good fabricator, and a good fabricator is usually a sign of the rest of the house.