Bastion Press' Fairies has stats for her, but I don't remember if it has a description of the diamond.
Actually, I'm using Bastion Press' Faeries as a baseline right now. I'm also mining Tad Williams' "shadowline" series of novels for some of my inspirations. The Queen of Air and Darkness, Titania, and Oberon aren't deities per se......they are very old, very powerful Feorin nobles......children of a true diety and a feorin. That gives me the ability to mix in some of the other, even older faerie such as Queen Maab, Herne the Hunter, Jack in Irons, The Fool etc. The Queen of Air and Darkness, Titania and Oberon are quasi deities as a result. They might be high level....between 25 and 40 lvls.......but not full fledged deities. I'm integrating some ideas about fey being hedged out by holy symbols, being barred from entering temples of mortal deities etc......it wouldn't make sense for them to have full deities if they could enter churches. I kind of see them on a similar level as demon lords or archdevils.
I figure that the Queen of Air and Darkness was corrupted by the black diamond......maybe it's done something "unnatural" by the standards of the faerie. Maybe it's given her some of the powers of a true diety (except for the ability to grant spells). That's kind of where I've been leaning.
I've been reading the work by dicefreaks and it's cool, bur it looks like the project has died as there have been almost no updates in a year. And they seem to have the faerie court as deities which is different from how I've envisioned them.
I'm playing with the idea of a love triangle between the queen of air and darkness, Oberon and Titania. Maybe the queen of air and darkness wanted Oberon (or titania) but was rejected and turned to this artifact the black diamond for the power it would give her, mistakenly thinking it would win her the love of the being she desired, but instead of getting her wish, it's evil corrupted her. She might have been chaotic neutral before but the artifact corrupted her completely to evil.
I just don't know what it would actually *do* aside from that.
So, while I try to figure that out, I've been starting up Baba Yaga.
Banshee