X cost CMC

[Destore117]

Do X costs count as 0 CMC or the amount of mana you pay for them?
Example
Play {Chimeric Mass} for {6} mana
On the stack would it be considered a spell of 0 CMC or 6 CMC? Also, it's CMC would retain when it hits the field correct?
Curious for cards such as  {Spell Blast} or  {Abrupt Decay}

[Giggle the Draco Genius]

Anywhere but on the stack spells with X are 0, while it is on the stack the CMC is equal to what X has been declared to be.

[ibtrickey]

202.3b: When calculating the converted mana cost of an object with an {X} in its mana cost, X is treated as 0 while the object is not on the stack, and X is treated as the number chosen for it while the object is on the stack.