What are the tables in Ab Jones’s bar made of?

The tables in Ab Jones's bar are made out of stone, gravestones, to be precise. This is just the kind of thing we might expect from Cormac McCarthy, with his heightened sense of the macabre. Ab Jones's novel use for gravestones is also expressive of his anti-establishment personality. He's an African American at a time and a place (1950s Tennessee) when racial prejudice was almost universal, and Ab's often on the receiving end of savage beatings from officers of the Knoxville Police Department. Like most African Americans of the time, he's subjected to bigotry and hatred on virtually a daily basis. It's not surprising, then, that he has such a deeply ingrained distrust of authority.
Ab's use of gravestones for bar tables is indicative of a society in which little of any real value is fixed or permanent. The graves are predominantly those of the poor and downtrodden, their graves washed away by the building of the great Tennessee dams. In his own unique way, Ab is preserving the memory of these long-forgotten folks. As a victim of daily oppression, he can empathize with these people and how they were ignored and disrespected by the authorities both before and after their deaths.