The Dark Ruby is one of the parts of Mirith's Royal Sceptre. Its full powers are unknown: Telchine claims that it is used to know someone's true nature. A series of experiments led in Mirith and its surrounding by several adventurers revealed that wizards (and only wizards) can use it to summon water, air, and fire elementals.
The Ruby was previously in the possession of the druid Fortinbras, until it was recovered by the Mirith Vanguard. Once all the parts of the sceptre were collected, the symbol of Mirithian power was re-forged, under the instructions of Avesne.