A wooden altar will have an altar stone imbedded in it. It may not be visible, though, if it is covered by the alter cloth. From Wiki, but accurate, “To provide for other circumstances—for chaplains of everything from military to Boy Scout units, for priests traveling alone, for missionaries, or for large outdoor celebrations of Mass on pilgrimages—portable altars, popularly called “altar stones,” were used. These were usually blocks of marble, often about 6 inches by 9 inches and an inch thick, consecrated as described above. A priest with a field kit could simply place this stone on any available surface (a tailgate, or a stump or log) to celebrate Mass, or it could be inserted in a flat frame built into the surface of a wooden altar. Many Roman Catholic schools had a full-sized, decoratively carved wooden altar (which, being wood, could not be consecrated) in their gym or auditorium that could be taken out and prepared for Mass, with an altar stone placed in the ‘mensa’ space.”
Good question.
Good points. I thought I might have seen a stone embedded in wood, but didn't consider so long as a piece of marble was consecrated (another important detail), that was interesting.