The cube's surface is pockmarked with jagged runes that seem to shift when unobserved.

Doubling Cube

{3}, {T}: Double the amount of each type of unspent mana you have.

The List

Tenth Edition

{3}, {T}: Double the amount of each type of mana in your mana pool.

Fifth Dawn

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

Doubling Cube's ability is a mana ability.

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