A dark stone is really the other side of the sacred seven. Like two sides of the same coin. A dark stone is something that has the ability of sacred seven. It does not have to be a human, but can also be an object. Hellbrick is an example. usually when a human is considered a Darkstone, they will unable to control their actions or can not control them for long. Knight is an example as he can usually control his Darkstone form with the aid of a vaccine which is made from the blood who has the activation ability of the Sacred Seven (i.e. Ruri, Aoi, or Fei).
Although during the majority of the anime, Aoi was petrified, Fei was considered "not pure enough" implying that her level of ability is not on the same level as Ruri. through process of elimination, this would mean the vaccine has a high chance of being produced from Ruri's blood
From the Darkstones that has been shown (Alma, Knight,and Kenmi in the second to the final episode), when ever they are about to transform into their Darkstone form, usually they would be clad in a black aura and either one or two red crystal horns will sprout from their forehead. Alma has shown that this transformation can be reversed, but it takes a lot of will power.
When a Darkstone devours a gem, it can apparently transform into a stronger form. In episode one, a Darkstone in the form of a headless statue of Perseus holding the head of Medusa was trying to get a gem, through a battle with Kagami, it lost its left arm that was holding the head of Medusa. after the main body was destroyed, the head regained mobility and while Alma was dealing with the body, the head made its way to the gem, devoured it, not only growing bigger, but also growing larger and more numerous snake heads.
A common ability between Darkstones and people who wield their true forms is that they both have the ability to sense one another presence.