Question:
I read in the Torah that Moses came down from the mountain with two tablets of stone,1 but did not see anything about their size, shape, or what kind of stone they were made of. Can you shed some light?
Response:
The Dimensions:
Tradition tells us that they were thick square blocks of stone, six handbreadths tall, six handbreadths wide, and three handbreadths deep. In modern measurements, that is just over 18”x18”x 9”. The sages of the Talmud demonstrate how tablets of this size--along with a few other relics--fit neatly into the Ark of the Covenenat that Moses made as described in Exodus.2
It is interesting to note that nowhere is there any mention of them having the rounded tops that are so common in the popular drawings of Moses and the tablets. This design appears to be the invention of non-Jewish artists.
The Material:
The tradition is that both sets of tablets were made of sapphire. After Moses broke the first set, G‑d revealed a large deposit of sapphire in Moses' tent. Moses used some of the stone to carve the second tablets and was permitted to keep the remains.3
The Writing:
The most common understanding is that the first five commandments were written on one tablet, and the other five commandments were on the second.4
The Torah describes the writing as "inscribed from both their sides; on one side and on the other side they were inscribed."5
This means that the inscription was engraved through and through. As such, the words were clearly legible on one side and written in mirror writing on the other. Now there are two Hebrew letters, the ם and the ס, that are closed from all sides. The centers of these letters, Rav Chisda concludes, must have been miraculously suspended in place.67
Others teach that the writing was miraculously legible on each side—in other words, although the letters were engraved all the way through, they could nevertheless be read from right to left on both sides. Rabenu Bachye explains that this is because the Torah can be understood on two levels, one revealed and one hidden.8
