Another cause is the proximity to iron metal which is oxidized by air in the.
Marble with iron oxidation.
In severe cases the process of oxidation is difficult to reverse or stop.
If you expose a brand new nail to water and air it will turn brown and rust.
The most common culprit for white marble turning yellow is iron which can be found in many natural stones.
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.
The same process is occurring with the iron in the marble.
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They are in the strict sense neither marble nor onyx for true onyx is a banded chalcedony composed largely of silicon dioxide.
Onyx marble was the alabaster of the ancients but alabaster is now defined as gypsum a calcium sulfate rock.
These marbles are usually brown or yellow because of the presence of iron oxide.
The same process is occurring with the iron in the marble.
When exposed to water acids or bleach the iron in the stone will begin to oxidize and turn it yellow.
The most common culprit for white marble turning yellow is iron which can be found in many natural stones.
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.
This process of oxidation is similar to the rusting of metal.
This oxidation process is accelerated when the tile is saturated as in the flood in the above example.
In extreme circumstances the marble can turn a brown color akin to what happens to a piece of iron left out in the rain.
When exposed to water acids or bleach the iron in the stone will begin to oxidize and turn it yellow.
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.
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 oxidation process is accelerated when the tile is saturated as in the flood in the above example.
This process of oxidation is similar to the rusting of metal.
I highly doubt it has anything to do with light sensitivity.