I'm running an all-dwarf campaign in which I've featured the remnants of a prehistoric pre-diefic dwarven mystery cult which venerated the earth and celebrated their race's ties to that elemental plane. During their adventures, the group has found a giant geode-like series of caverns hidden beneath the First World in which dwarves, dwarf-oreads and earth elementals members used magic to record memories into the countless crystals lining the walls; a great archive of sorts.
The PCs are using these crystals to learn the cult's lost secrets. I'm wondering what sorts of things they might be able to glean from perusing the archive. Earth-based spells are a given, but I'm wondering if there are any feats, archetypes, prestige classes or other things that the players themselves may consider desirable. I'm open to 3rd party and even 3.X material if it's somehow earth-based and appropriate. The PCs are a Stonelord Paladin, a Warpriest, a Skald, an Invulnerable Rager Barbarian and a Conjuration Wizard.