Extra Ores

Extra Ores is a mod included in the Narchnic Pack which does Exactly What It Says On The Tin (warning: tvtropes).

The ores added include:
 * Silver -- about 2.5 times as common as gold, and spawns between 0 and 32 world height.
 * Used by an alternate IC2 recipe for Glass Fibre Cable.
 * Ruby, Sapphire, and Emerald -- gems that make tools which are about as good as diamond, except they only last for 500 uses.

As of Narchnic v1.2.4.4 (ExtraOres v0.0.4), the mod also adds geodes, which are underground spherical rooms with walls made of precious, precious ores (here's an example from an early version; note that glowstone is somewhat rarer in the current version). Geodes are fairly rare (currently: a 1/100 chance to appear in a given chunk), and if present will appear at bedrock level.