Salaam sister,
From Shariah perspective, it is reported in several hadiths that the Prophet (pbuh) used to prostrate on the ground.
Abu Sa’eed used to relate that he had seen the Prophet prostrating on wet mud, so much so that he could see mud stains on his forehead.
Saheeh Bukhari, Part I, p. 104
The Prophet used to place his forehead on the earth while prostrating.
Saheeh Bukhari, Part 1, p. 97
The Prophet declared that the best place for prostration was the earth, or upon something that grows from the earth.
Kanz-ul-Ummal, Part 4., p. 113
There are many more hadiths and proofs about this. Therefore, the followers of the Jaafari school of thought deem it necessary to perform the sajda on “earth, and on those things which are not edible nor worn, and on things which grow from earth (e.g. wood and leaves of trees)”. So if one is praying outdoors then sajda can be done straight on the earth. But if a person is indoor and is unsure of the tahara of the floor then a piece of clay/sand/leaf is used to perform sajda. However, it is a common practice in Shias to carry a little clay tablet (what we call in Pakistan as Sajda-gah whereas the Arabs call it Turba) that can be taken out and used at the time of salat. Most of the times the clay used to make this tablet is sand from Karbala, land where Imam Hussain (as) got martyred with his family and companions.