Doubling Cube's ability is a mana ability.

Any restrictions on the unspent mana aren't copied. For example, if you have with no restrictions on it and that can be used only to cast artifact spells, you'll end up with , that can be used only to cast artifact spells, and that can be used for anything.

The "type" of mana is its color, or lack thereof (if the mana is colorless).