Not sure that marble would withstand the heat of an oven set at 500 degrees.....and a cool pizza slid on top. Besides it's not porous like a pizza stone to absorb moisture and make the crust crisp.
I would be afraid it would split...in fact I would almost bet it would on the first try.
Linda C

Ditto, what Linda said! Tell her to put it in a place where it won't be moved and use it for pastry making, rolling out cookie dough, candy making, etc.

I would not use it unless it had no fissure lines. Pizza stones are made of ceramic and are tempered for the heat - they are not made of real "stone". Limestone might work, but I would never attempt it with any natural stone.

Lars

Not to mention the staining that would occur to the marble. It just was not made to be used as a baking stone. NancyLouise

I have not had any experience with Marble, i'm guessing it would take quite some time to heat up.

I would use the slab of marble for kneading and rolling dough, I don't think it would be useful in the oven.

I have a piece of granite which is designed to be heated in the oven and then placed in it's holder and used to cook tableside, hors d oevres, for example. I leave it in the oven and use it for baking breads, as a pizza stone.

I do not know what chemicals are used to finish the marble in question and that would be important.

I don't think marble, per se, would be a problem, but whether the finishes are food safe. I wouldn't be concerned with cracking.

There is place, near here, that does stuff with ice cream,
on a surface of ice cold marble, (Stone Cold Creamery),
But I don't think it would work in an oven.

Marble per se....which means "of itself"...is definitely a problem...most marble is unfinished...just polished.
Marble is dolomite....calcium carbonate, acid, like tomatoes erode it
Marble is s "soft" stone....compared to granite....and compared to a traditional pizza stone non absorbent....won't accomplish the crisping of the crust that a porous stone will.
However marble, because of the afore mentioned properties makes a lovely pastry stone. Don't ruin it by putting it into an oven and trying to0 cook a pizza on it.

Pizza/baking stones made of unglazed ceramic clay are the most preferred.

Some prefer using marble, but one of the drawbacks is that they are not that easy to take out of the oven for cleaning because they are much heavier. Marble is also usually more costly.

Here is a link that might be useful: Baking Stone Tips and History

Op asked how a piece of marble would fare if used as a pizza stone, in an oven. Marble does make a lovely pastry stone.

Acid, like tomatoes, will etch the marble surface but not erode it. Hundred year old marble exposed to the elements is not eroded. I have honed marble countertops.

Marble is likely polished with chemicals.

Pizza stones are porous and absorb water. But I do not know the science behind crisp crusts, other than steam injected ovens are used in baking breads to crisp crusts.

I use a piece of granite in an oven for baking breads on parchment paper. The granite is sold as a cooker for tableside foods. Amazon has something similar for raclette.

Wondering where you got the idea that marble will not be eroded by acids? Ever seen a marble statue that has been exposed to acid rain?
Marble is polished with a fine grit. Michelangelo's Pieta was not polished with chemicals.

I, too, am wondering where you got the idea that marble will always be eroded by acids. Let's put this in the perspective of the issue of marble as a pizza stone for an enquirer who will not likely live over one hundred years.

My marble counters will not be eroded by acids in a hundred years. Pompei is in pretty good shape.

I have honed marble countertops which are in contact with acidic foods, among others. I try to protect with cutting boards, but don't catch everything. There are no stains but there are some etches. The etches can be polished and restored to original condition. Erosion, no. Faint removal of finish, yes, that's an etch.

Acid rain in a kitchen? Inside an oven?

Is the treatment of Michelangelo's Pieta relevant to the query of marble as a baking stone?

All I was saying was that I have a piece of granite being used as a baking stone.

Marble cookware, which are cookware items that are coated with marble are actually considered safe. Meaning they don't contain harmful chemicals.

Marble however, needs to be cleaned more often because it easily gets stained. On the other hand, baking stones really don't need to be cleaned that often. So again, if you decide to use marble as a pizza stone, the heaviness of it could be an inconvenience for taking it out of the oven and cleaning it.

One good thing about marble though, is that you could use it as a pastry board.

Here is a link that might be useful: Marble Cookware

Marble cookware is non-stick coated with ceramic marbles....whatever that is. Not actual calcium chloride.
Marble is calcium chloride and will be dissolved by vinegar. Call it eroded...etched whatever you want...but acid will dissolve marble. Crush a piece of marble and soak in vinegar overnight, and it's gone.

Hundreds of years old marble statues exposed to the atmosphere lose their noses....and other detail.

If she wants a pizza stone that would be easier to care for and does not want her marble stone how about asking someone if they would like to trade. I am sure there is someone that would love to have a nice piece of marble for a candy and pastry board.