עמידה של שבת – ושני לוחות אבנים הוריד בידו
It is not explained why the need to emphasize in this Tefillah that the Luchos were specifically made of stone. The content (the message) of the 10 commandments is what is important. The container (whether stone, metal or whatever material) was not important at all. This concept is similar to the Medrosh that the Torah is beautiful even in an “ugly” container.
In fact it would be proper to describe the Luchos as לוחות העדות (like in Devarim 9,9).
However, it is wonder why Moshe (Devarim 9, 10) calls it also Luchos of stone (several times). And even HaShem (Shemos 24, 12) call them לוחות האבן. Even in future generations, in the time of King Shlomo (Kings I 8, 9) they would also refer to the Luchos of stones.
The Medrosh Rabbah (כי תשא) offers an explanation that the Luchos were referred to as stones, since the most common punishment for the Bais Din is to apply a death penalty of stoning. However, Rabbi Epstein maintains that this explanation is not adequate since the places that mention the Luchos of stones appear to be for the sake of good and loving contexts (and not punishments).
Possibly, the intention of calling the Luchos of stones is not to call attention to the type of materials of the physical Luchos but to convey the concept of a foundation of the purpose of the Luchos, like a cornerstone of a beautiful building.
Thus, the adjective of stones alludes to the 10 commandments that are spoken about in the Luchos, which are the foundation to the entire Torah.
