The staff provides unlimited runes in the element it is set to. You can change the element of the mindspike by right-clicking on it and selecting "Change element". The staff's current element is shown in brackets at the end of the item name (e.g. Mindspike (air)). You can change the element back and forth with no tangible limitations. This can be used to save inventory space when casting spells that require different types of runes. The mindspike has a special attack, as do its melee and ranged equivalents, the keenblade and quickbow.