Indeed it was *[klaf][1]*, the same as a sefer Torah.

The Rambam writes in [Hilchot Sotah 3:8][2]

> Afterwards, a scroll of parchment from a kosher animal, like the
> parchment used for a Torah scroll is brought.

R Eliyahu Touger [explains][3] the Jerusalem Talmud (Sotah 2:4) states that the parchment must be made from the hide of a kosher animal, lest the woman refuse to drink and the passage be required to be entombed. It would not be fitting for God's name to remain on parchment from a non-kosher animal.

See also [Hilchot Sotah 4:8][4] ("not to write on paper") and [Hilchot Tefilin 1:10][5] ("hides of [all] kosher animals, wild beasts, and fowl").


  [1]: https://en.wikipedia.org/wiki/Klaf
  [2]: https://www.chabad.org/library/article_cdo/aid/960640/jewish/Sotah-Chapter-Three.htm
  [3]: https://www.chabad.org/library/article_cdo/aid/960640/jewish/Sotah-Chapter-Three.htm#footnote21a960640
  [4]: https://www.chabad.org/library/article_cdo/aid/960641/jewish/Sotah-Chapter-Four.htm
  [5]: https://www.chabad.org/library/article_cdo/aid/925417/jewish/Tefillin-Mezuzah-and-Sefer-Torah-Chapter-One.htm