Generally speaking, gambling and playing with gambling instruments are haram. The prohibition of card game which is considered to be a means for gambling is also for the same reason. Therefore, playing a game with anything which is considered an instrument of gambling is forbidden.
To explain further, there is a difference between gambling and playing with its instrument. It is possible that a person might gamble without the involvement of any gambling instrument. However, the prohibition of a game is not because of gambling only; in fact, playing with the means and instruments of gambling is also forbidden as per the narrations and verdicts of religious authorities (grand jurists).
It should be noted that a gambling instrument is an instrument which is normally used for gambling. In other words, the general rule is that it is made for gambling and used for it, such that the term "gambling instrument" can be applied to it. It is enough if it is regarded as an instrument (for gambling) by the faithful and religious adherents even though some may use it, in some cases, for playing without money but for entertainment or for sharpening the mind. The (card) game is a game played with the means for gambling and it is forbidden even if we do not consider it as "gambling".
In any case, grand Ayatollah Sistani's fatwa about card game is as under:
It is not permissible, as an obligatory precaution, to play with instruments that are considered according to the custom of the locals to be a means for gambling even if it does not involve any betting. The terms "custom of the locals" means that the instrument is normally used for betting in that region.
Question 5711 (site: 5989) Index: Card game, a means for gambling.
Question 3539 (site: 3777) Index: Playing with card game without betting.