Aztec Calendar Stone
In the midst of renovating his home, the OP’s grandfather warned him not to remove this ancient-looking object from the wall. After asking gramps what it was, he was just as confounded as the OP. So, without anywhere else to turn to, he went straight to Reddit.
One user said that this is an Aztec Calendar Stone, which the ancient civilization used to tell dates. Some Calendar Stones can measure as wide as 8 feet and weigh up to 25 tons. This one, however, was a souvenir that the grandpa bought in his travels to Mexico. Sure, it doesn’t have any historical value, but there is some sentimental value to it.