What was the ark of the covenant made of?

Answer: Acacia (shittim) and gold

The ark was made of acacia wood (KJV: shittim) and overlaid with pure gold inside and out, with a gold molding around it.

“And they are to construct an ark of acacia wood, two and a half cubits long, a cubit and a half wide, and a cubit and a half high. Overlay it with pure gold both inside and out, and make a gold molding around it.”— Exodus 25:10-11 (BSB)