If you expose a brand new nail to water and air it will turn brown and rust.
Marble with iron oxidation.
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In addition to the oxidation of internal iron compounds present in stone like pyrite fes 2 and siderite feco 3 1 2 contact with iron rich ground water when marble is used in for example garden fountains results in severe and unsightly discolouration.
The most common culprit for white marble turning yellow is iron which can be found in many natural stones.
This oxidation process is accelerated when the tile is saturated as in the flood in the above example.
They are in the strict sense neither marble nor onyx for true onyx is a banded chalcedony composed largely of silicon dioxide.
I highly doubt it has anything to do with light sensitivity.
Oxidation is responsible for yellow brown stains that look like they are coming from the inside out.
The same process is occurring with the iron in the marble.
This process of oxidation is similar to the rusting of metal.
When exposed to water acids or bleach the iron in the stone will begin to oxidize and turn it yellow.
In severe cases the process of oxidation is difficult to reverse or stop.
Another cause is the proximity to iron metal which is oxidized by air in the.
When exposed to water acids or bleach the iron in the stone will begin to oxidize and turn it yellow.
The same process is occurring with the iron in the marble.
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.
This process of oxidation is similar to the rusting of metal.
The rust stain simply returns as moisture brings more rust back to the surface.
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.
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.
This oxidation process is accelerated when the tile is saturated as in the flood in the above example.
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.
The most common culprit for white marble turning yellow is iron which can be found in many natural stones.