The most common culprit for white marble turning yellow is iron which can be found in many natural stones.
Marble with iron oxidation.
Within marble is a naturally occurring percentage of iron and if the stone is exposed to high quantities of water a flood or leak or continuous moisture as inside a shower the iron will literally rust.
These marbles are usually brown or yellow because of the presence of iron oxide.
When exposed to water acids or bleach the iron in the stone will begin to oxidize and turn it yellow.
Another cause is the proximity to iron metal which is oxidized by air in the.
They are in the strict sense neither marble nor onyx for true onyx is a banded chalcedony composed largely of silicon dioxide.
In extreme circumstances the marble can turn a brown color akin to what happens to a piece of iron left out in the rain.
This process of oxidation is similar to the rusting of metal.
Oxidation is responsible for yellow brown stains that look like they are coming from the inside out.
If you expose a brand new nail to water and air it will turn brown and rust.
The same process is occurring with the iron in the marble.
This oxidation process is accelerated when the tile is saturated as in the flood in the above example.
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I highly doubt it has anything to do with light sensitivity.
In extreme circumstances the marble can turn a brown color akin to what happens to a piece of iron left out in the rain.
The most common culprit for white marble turning yellow is iron which can be found in many natural stones.
If you expose a brand new nail to water and air it will turn brown and rust.
The same process is occurring with the iron in the marble.
The rust stain simply returns as moisture brings more rust back to the surface.
Yellow brown stains in white marble like carrara marble are almost always due to the oxidation and rusting of iron deposits that are embedded in the marble as described in this article above.
This process of oxidation is similar to the rusting of metal.
Onyx marble was the alabaster of the ancients but alabaster is now defined as gypsum a calcium sulfate rock.
This oxidation process is accelerated when the tile is saturated as in the flood in the above example.
Iron stains however can be eliminated by mopping away water as accumulated in the marble tiles in order to prevent the iron from getting oxidized.