According to Exodus 31, the two covenant tablets were made from stone.
The chapter says that God gave Moses two tablets of the covenant, written with the finger of God. They are described as tablets of stone. These tablets contained the testimony associated with the covenant made at Mount Sinai and were placed in the Ark of the Covenant according to the Tabernacle instructions.
The stone tablets are separate from the wooden Ark that carried them and from the gold used to overlay or furnish sacred objects. Exodus later says that Moses broke the first tablets when he saw the Israelites worshiping the golden calf. A second set was then made after the covenant was renewed.
The passage does not specify the geological type of stone. It also does not give the tablets’ dimensions. Consequently, answers such as granite, marble, or a particular size go beyond what Exodus states. The canonical material is simply stone.