Vivacite ("vivacity stone") is emerald or peridot (or any similar-looking greenish stone) believed to be infused with the physical energetic force (stamina) of a person (or other sentient being), and can be used to absorb and transfer stamina to and from humans. It is a type of enchanted gemstone in that peoples' beliefs that they hold stamina gives it that property. The larger the vivacite, the faster it can absorb or dispense stamina, allowing the user to achieve greater feats of superhuman strength and agility, and the more stamina it can hold.
It is believed to be charged by stabbing into a person. The vivacite then absorbs the stamina from them, stripping them of stamina in the process, until they have no stamina left for anything more than breathe and swallow and blink. This also means that vivacite can be used to tender a person physically helpless. When charged, vivalite glows with a greenish light.
Vivacite can be ground up and immersed in liquid (oftentimes alcohols, oils, or honey) to make stamina 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 stamina, the presence of the stamina increases the person's "stamina pool", and in a way that cannot be readily stripped away by others.
Vivacite can also be worn, such as in jewelry. In this case, a belief of ownership - that one owns the stamina - is enough to allow one to tap the stamina in that vivacite; in other words, it need not require physical contact with the skin. Alternatively, vivacite 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 vivacite they're wearing is theirs, and not stolen).