It's not stated explicitly, but limestone or marble seems the best fit
I'm not aware of any material that explicitly answers this, but the material components for the flesh to stone spell are "lime, water, and earth."
"Lime" is calcium oxide or calcium hydroxide, typically manufactured from limestone and mixed with water and sand/gravel to make concrete. Already limestone seems like an obvious fit.
Statues are commonly made out of marble… and that happens to be a metamorphosed form of limestone. This is also a strong contender.
Limestones and marbles are based primarily on calcium. As well as being present in the material components, it's also found in the human body in large quantities (in bones, in teeth, and as an important electrolyte). Humans are about 1.5% calcium by mass.
On the other hand, most other rocks involve significant quantities of silicon, but that's only a trace element in the human body (<0.1% by mass), and silicon isn't listed in the material components. ("Earth" could be just about anything from gravel to mulch; some would contain silicon, others wouldn't.)
So, going by both the material components and the human body, we're probably looking at a calcium-based rock without large amounts of silicon, which implies limestone or marble. It could even be concrete—it's a rock, though not a very romantic one.
I'm told (h/t Molot) that "In one of the Underworld books (prose, not rules) drows turned victims into onyx". While true onyx is a silicate, most "agate" sold today is in fact calcite (calcium carbonate), commonly found within limestone, so this is still consistent with the "calcium-based rock" interpretation.
If you want to enforce conservation of mass, then this fine answer's suggestion of pumice is definitely the best option, but spells are generally not known for respecting conservation of mass. For example, Iron Body multiplies a person's weight by ten.
The Petrified Condition would not be suppressed or removed unless it is magical
Antimagic field has a specific list of effects stating what it is capable of doing - and the one most applicable to this situation would be:
Targeted Effects. Spells and other magical effects, such as magic missile and charm person, that target a creature or an object in the sphere have no effect on that target.
Petrified is neither a Spell effect or a Magic effect, but a condition, similar to being grappled or exhausted.
If you wanted to remove petrification, you need to use a spell that specifically states it is capable of doing so. For example, the spell Greater Restoration:
You can reduce the target's exhaustion level by one, or end one of the following effects on the target:
- One effect that charmed or petrified the target
However, when the effect is applied by a Beholders eye rays, it is considered magical:
Eye Rays. The beholder shoots three of the following magical eye rays at random
Therefore - magical petrification would be unable to affect a target within the sphere, and both the on-going saving throws & petrification condition itself would be supressed if the targets enter it while affected.
Does suppressing petrification stop the on-going petrification effect?
If the petrification is considered magical, it will be suppressed, both by Antimagic field & the Beholders own Anti-magic eye cone.
On a failed save, the creature begins to turn to stone and is restrained. It must repeat the saving throw at the end of its next turn. On a success, the effect ends.
If the effect is suppressed, the target doesn't make a saving throw, but it also states that the effect is only ended on a success. Therefore the effect continues until the target succeeds on a saving throw.
Best Answer
An antimagic field suppresses Flesh to Stone during its duration and prevents the final petrification (but doesn't reverse it after the fact)
During the 1-minute duration of the Flesh to Stone, all of the spell's ongoing effects are magical and would be suppressed if the target were put inside an antimagic field. This means that the target would not be restrained or petrified and will not have to make any saving throws against the spell while inside an antimagic field. Among other things, this means that the target will not accumulate any successes or failures on saving throws.
The more interesting question is what happens at the end of the 1-minute duration:
Normally, the petrification becomes permanent if the spell's full duration elapses; since the spell has ended and nothing about petrification is inherently magical on its own, at this point the petrification becomes non-magical. Notably, the permanent petrification still occurs even if the creature has not yet failed 3 saving throws and becomes (temporarily) petrified by the time the spell ends, a situation that is normally impossible without an antimagic field.
However, even though the petrification becomes permanent and non-magical after the end of the spell's duration, the transformation itself is magical. Hence, if the creature is inside an antimagic field at the time the spell's duration elapses, the permanent transformation never occurs in the first place, and the petrified condition is never inflicted. So, as long as the target is inside the antimagic field when the spell ends, they walk away without any lasting effects from the spell.
Finally, if the target has already been permanently petrified by a Flesh to Stone spell, there is no remaining magical effect to suppress, so putting the petrified creature in an antimagic field has no effect - the creature remains petrified.