Praise be to Allah.
The “Scriptures of Ibraaheem” were Scriptures that Allah, may He be exalted, sent down to His Prophet and Close Friend (Khaleel) Ibraaheem (blessings and peace of Allah be upon him). They contained a number of exhortations and rulings.
They are among the divinely-revealed scriptures that we are obliged to believe in. Allah, may He be exalted, says (interpretation of the meaning):
“The Messenger has believed in what was revealed to him from his Lord, and [so have] the believers. All of them have believed in Allah and His angels and His books and His messengers, [saying], ‘We make no distinction between any of His messengers.’ And they say, ‘We hear and we obey. [We seek] Your forgiveness, our Lord, and to You is the [final] destination’”
Shaykh Ibn Baaz (may Allah have mercy on him) said:
We believe in all the Books of Allah, in general terms and in detail. We believe in all the Books that were sent down to the Messengers (blessings and peace of Allah be upon them), including the Tawraat (Torah), Injeel (Gospel), Zaboor (Psalms) and Qur’an, as well as the Scriptures of Moosaa and the Scriptures of Ibraaheem. We believe in all the Books that Allah sent down to His Messengers.
End quote from Majmoo‘ Fataawa Ibn Baaz (28/17).
See also the answer to question no. 126004.
With regard to the nature of these Scriptures and what they were written on, and whether they were written on tablets, leaves or animal skins, this is something that no one knows except Allah, and we have no information about it. Knowing that is not connected to any aspect of faith or knowledge; knowing it does not matter and inquiring into it is no more than a waste of effort.
What Allah, may He be exalted, has told us is that they were Scriptures (suhuf, singular saheefah). The basic meaning of the word saheefah is what is spread out of anything, so the saheefah is something that is spread out, whether it is a leaf or sheet, or an animal skin, or a tablet, or anything else that may be written upon.
At-Tabari said in his Tafseer (24/377):
The word suhuf (scriptures) is the plural of saheefah; what is meant here is the Books of Ibraaheem and Moosaa.
Ibn Manzoor said in Lisaan al-‘Arab (9/186):
A saheefah is something that is written on; the plural forms are sahaa’if, suhuf and suhf. In the Qur’an it says (interpretation of the meaning): “Indeed, this is in the former scriptures, The scriptures [suhuf] of Abraham and Moses” [al-A‘laa 87:18-19] – that is, the Books that were revealed to them, blessings and peace of Allah be upon our Prophet and upon them.
It is strange that the questioner asks about what these Scriptures were made of, then he says, “regardless of their contents”! If he had asked about their contents, then said “regardless of their [physical] nature,” that would have been more appropriate and more beneficial.
And Allah knows best.