White marble such as Carrara marble can have embedded iron deposits.
Rust stains on white marble often result from oxidation of these iron deposits from repeated water exposure such as in a shower, around a faucet or toilet. This can also occur on marble floors from flooding, ground water seeping up through the floor tile or from a plumbing leak.
Rust stains that occur from metal objects on the surface typically have the shape of the object that rusted and the rust stain is more orange. This type is on the surface and can be removed using the Granite & Marble Rust Stain Remover when reasonably fresh.
Rust stains from embedded iron deposits in white marble are usually more muted yellow-brown in color and more diffuse without distinct shape. In such cases, the rust stain may spread until the source of water that is absorbing into the marble and oxidizing the iron deposits is stopped.
Another way a rust stain can appear on shower tiles is if metal pieces were used in the construction of the shower. If these pieces are not sealed well, then water can get to them, oxidize the metal and then the rusty water evaporates through the tile leaving the rust stain.
Usually, rust stains in white marble are due to embedded iron deposits, but any metal under the tile can lead to a similar diffuse rust stain.
This type of rust stain is nearly impossible to remove since the source of the stain is within or ran through the marble creating a deep and extensive stain which occurs over a long period of time. After removing the source of water that is oxidizing the embedded iron deposits it may be possible to lighten the stain with repeated application of the Granite & Marble Rust Stain Remover, but again deep, old and/or extensive rust stains are usually impossible to remove completely even with professional help. Replacement is often the most effective option.
See this Rust Stain Removal article for more details.