Part of the issue in trying to decipher the truth about the Ark of the Covenant, even the truth of its existence, is all the secrecy and protectiveness surrounding the artifact. No one was allowed to tend to it besides the ancient Israelite's Levitical priest class. It's apparent, current resting spot in Ethiopia, in St. Mary of Zion Church, is tended by a single monk called (this is true) "The Guardian," who himself is not allowed to see the Ark, as described in Slate. 

One thing can be agreed on: the Ark is supposed to contain the Ten Commandments, the essence of "the law" given to God's chosen people. You know, "thou shalt not kill," "thou shalt not steal," etc. That is, the Ark houses the physical stone tablets onto which the commandments were scrawled (the second copy, that is; Moses broke the first copy — it's a long story). In the classic 1956 movie Ten Commandments, God shoots some fire at the tablets and, in a feat of pretty sweet animation, burns his words onto them, as seen on YouTube (the "finger of God" in Exodus 30:18). It's safe to assume that Moses most likely used a chisel, though.

Per the English Standard Version, in Hebrews 9:4, the Bible does go into some detail about the Ark's contents, "in which was a golden urn holding the manna, and Aaron's staff that budded, and the tablets of the covenant." 

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