If used as such, yes they are. In other cultures things such as beads, neck rings, shells have all been used as currency. In modern times diamonds are used as currency, and in some illicit circles so are paintings or other artwork.
Paintings aren’t used as currency, they are used as assets or stores of value.
Similarly, diamonds are pretty rarely used to actually transact and are rather just asset stores. It goes cash->diamond->cash; not cash->diamond->something else.
Anything can be traded or bartered that doesn’t make it currency. Some cultures used beads and shells and stuff but don’t any more. That doesn’t make beads currency.