Ebony is the darkest level a Jewel can possibly be. It is a level darker than Black and a level deeper in what the Blood call the Abyss.
The only person to ever obtain an Ebony Jewel was Jaenelle Angelline . At her birthright ceremony Jaenelle was given thirteen Black Jewels as well as one Jewel of every color before it. When she made her Offering to the Darkness she obtained six Ebony Jewels.
There is no single power that can best that of Ebony. During Jaenelle's reign as Queen of the Dark Court she gave to her first circle Rings embued with Ebony power that would allow them to erect an Ebony shield around themselves. Even after the Ebony was lost in Jaenelle's purge of the Tainted witches the power in the rings remained but was only used in severely dangerous instances.