https://en.wikipedia.org/wiki/Ark_of_the_Covenant
from link:
“...gold-covered wooden chest with lid cover described in the Book of Exodus as containing the two stone tablets of the Ten Commandments. According to various texts within the Hebrew Bible, it also contained Aaron’s rod and a pot of manna.[3] Hebrews 9:4 describes: “The ark of the covenant [was] covered on all sides with gold, in which was a golden jar holding the manna, and Aaron’s rod which budded, and the tablets of the covenant.”[4]”
They are talking about the altar, not the Ark.
Sounds like a piece of masonry.