Prostration (sujud)

Bismihi ta'ala 

If it is natural marble, and has not been laminated by plastic or anything else, then it would be permissible to do sajdah on.

And Allah knows best.

Bismihi ta'ala

No, it would not be valid. 

And Allah knows best.

Bismihi ta'ala

Hijab for women is the covering of everything, except for the face and hands. As far as when a woman is praying, she must also wear complete hijab, whether she is in public or she is all alone.

However, our esteemed jurists mention that the feet are exempted from being covered while praying. This means that when a woman is praying, it is not wajib for her to wear socks or cover her feet, unless she is in public or there is a non-mahram male who can see her. 

And Allah knows best.

Bismihi ta'ala

Although there are certain things haram for a woman to do while she is in her menstrual cycle, like pray or recite certain parts of the Quran, but it is still permissible, or rather recommended for her to do worship. 

It is mustahab at times of salat for her to sit at her prayer mat and do dhikr, or recite dua, etc. 

It is also permissible for her to perform sajdah, like sajdah shukr. 

And Allah knows best. 

Bismihi ta'ala

There are seven obligatory parts that must be touching the ground while in the state of prostration in Salat.

Tip of the toes of both feet, both knees, both palms, and forehead. 

It is also mustahab for the nose to be touching earth as well.

And Allah knows best.

Bismihi ta'ala

The dhikr that should be recited in sajdatay al-sahw are:

بِسْمِ اللهِ وَ بِاللهِ، اَلسَّلَامُ عَلَیْكَ أَيُّـهَا النَّبِيُّ وَ رَحْمَةُ اللهِ وَ بَـرَکَاتُهُ

bismillāhi wa billāhi, assalāmu ʿalayka ayyuhan nabiyyu wa raḥmatullāhi wa barakātuh.
In the name of Allah and by Allah. Peace be upon you O Prophet, and Allah’s mercy and His blessings (be upon you too).

I also have a tutorial that could be of benefit as well:

And Allah knows best.