How many Zaydi Madhabs are there in general?

 That is discouraged because out of humility to Allah, one prostrates upon the earth with their faces and forehead. Because prostration is only out of humility to Allah, Glorified be He, lowliness is between the slave and Allah, Majestic and Mighty be He. If one were to pray upon those things that you mentioned, the prayers are not invalidated nor does one have to repeat them.

It is preferable to me that the one praying place his forehead on the dust or the lowest part of the earth. It is necessary that one prostrate on that which grows from the earth, like herbage and the like. The exception is if one fears injury from extreme heat or cold. Then, one should be God conscious by what he is wary of and prostrate on what he likes.

I do not prefer that one prostrate on anything other than that which grows from the earth. It is said that whatsoever is permissible to pray in is impermissible to prostrate on. Precaution regarding that is preferable. According to me, the best thing is that one places the forehead on the low part of the earth or any of the herbage from it, such as the papyrus plant, as well as what grows in it like old cotton (al-kursuf), linen (al-kattān) and other herbs.

According to our imams, it is permissible to prostrate on a turbah provided that one does not believe that there is inherent virtue in such (Ref. Al-Jawaab al-Kaashif).