Manacite ("mana stone") is quartz, diamond (or any similar-looking clear stone) believed to be infused with mana, a calculable amount of magical potential energy, and which can absorb and transfer this mana to and from humans. It is a type of enchanted gemstone in that peoples' beliefs that they hold mana gives it that property. The larger the manacite, the faster it can absorb or dispense mana, and so the greater the magic spells that can be worked with it, and the more mana it can hold.
It is believed to be charged by stabbing someone for a short duration, in which case it rapidly drains away all of that person's mana possessed at the time, and drains it much faster than the person can regenerate it. Hence, this can be used to prevent a person from working magic using "innate mana". A person can also store their own mana in this way, and/or trade it for other goods.
Manacite can be ground up and immersed in liquid (oftentimes alcohols, oils, or honey) to make mana potions. Alternatively they can be swallowed directly, but a large enough stone may cause choking (which may result in death). In addition to giving the consumer an immediate infusion of mana, the presence of the mana increases the person's "mana pool", and in a way that cannot be readily stripped away by others.
Manacite can also be worn, such as in jewelry. In this case, a belief of ownership - that one owns the mana - is enough to allow one to tap the mana in that manacite; in other words, it need not require physical contact with the skin. Alternatively, manacite can be worn or attached to wielded items, such as staves, to strengthen the belief of ownership (since one can more readily convince oneself that a certain manacite they're wearing is theirs, and not stolen).