Rock Inscriptions, because they are made out of rocks which don't disintegrate or deteriorate over the centuries, are excellent pieces of evidence from the 7th century to give us a window into what was happening in that part of the world and at that time.
So far researchers have located over 100,000 Rock inscriptions with 30,000 of them catalogued and surveyed across Arabia, the Negev, the Trans-Jordan, and Syria. Another 70,000 more have yet to be catalogued.
One would expect Muhammad’s name, or references to Islam, or to Muslims, or even references to the Qur'an and Mecca on these inscriptions, all of which are located along the Haj routes, yet we don’t find one inscription with any of these references on them until after 690 AD, a full 60 years after Muhammad supposedly died.
The Arabic used in these inscriptions are all chiselled in the Nabataean-Aramaic script which was the same Arabic which is later used in the earliest Qur'anic manuscripts. If they had been created in the Hijaz part of Arabia (i.e. the central Western area), as Muslims tell us, they would have used the southern Sabain script which was used in what is today Yemen and Oman, a script which had been created in 600 BC, and continued for 1200 years in southern and central Arabia.
Ironically, the Sabaic script contained all the needed vowels and consonants required for a religious text, unlike the 7th century Arabic text which was used for the earliest Qur'anic manuscripts, which only employed between 14 - 16 consonantal letters, but without any of the 5 diacritical dots, nor the 3 vowels, all of which had to be created in the 8th and 9th centuries in order for people to understand what they were reading in the Qur'an.
Furthermore, in the 7th century there were no rock inscriptions in the central part of Western Arabia, because there were too few people living there, due to the desert environment of that area. Without water, you cannot have many people, and without large numbers of people, just like the coins, you aren't going to have many rock inscriptions.
Dr Ilka Lindstedt, in his research of rock inscriptions between the years 640 AD - 740 AD (which is roughly the Umayyad Period) found that almost all of the 7th century rock inscriptions were located in the north (i.e. around what is today Jordan, Israel, Lebanon, and Syria), and in the far south (i.e. around what is today Yemen).