Gaijin Hunter seems to have something on melding:
Mystery Melding
Makes 1 Mystery Charm.
Cost: 1 Frenzy Shard + 3 Pawn/Bishop Talismans
Luminous Melding
Makes 1 Shining Charm.
Cost: 1 Frenzy Crystal + 3 Knight/Rook Talismans
Ancient Melding
Makes 1 Timeworn Charm.
Cost: 1 Vile Frenzy Crystal + 3 Queen/King/Dragon Talismans
Fabled Melding
Makes 1 Enduring Charm.
Cost: 1 Pure Frenzy Crystal + 3 Hero/Legend/Creator Talismans
Halcyon Melding
Makes ??? number of ??? Charms.
Cost: 1 Frenzy Shard, Crystal, Vile Crystal + 3 Pawn~Unknowable Talismans
Juju Melding
Makes ??? number of Timeworn/Enduring Charms.
Cost: 3 Vile/3Pure Frenzy Crystals + 3 Pawn~Dragon Talismans.
Based on that information, it looks like melding works on their respective "tiers". Throw in three of the same tier, get another of that tier.
As for what you get out of it, yeah, that's completely random. So far as I've experienced, and been able to tell, charms can roll with any skill, at any value. I've gotten a +10 Cold Res Mystery Charm, while in 3U, I've gotten an Eating +1 on a Rare10 charm. There's no rhyme or reason, so what you get out of it is a crap shoot.
My gut says that the higher the tier, the better the chance of a higher level skill, like Expert, and the better the chance of a higher value, but I have absolutely nothing to back that up.