Obsidium Ore is a new mineral that is being introduced to mining in the Cataclysm Expansion. Prior to it’s release as a common mineral in this expansion it was previous a key drop after killing mobs in old world zones such as the Ruins of Ahn’Qiraj.
When the Anubisath mobs in Ahn’Qiraj were killed a small or large obsidian chunk could fall to the ground was was minable with 305 mining. The chunk would give obsidian shards, gems, or scarabs. It required a player to have +5 mining enchant and maxed mining at skill level 300 in order to mine it.
Now it is being introduced back into World of Warcraft Cataclysm as a common mineral. When mining Obsidium Deposits a player may find it accompanied by Volatile Fire and Shadow.
Obsidium Deposits will appear in the beginning Cataclysm zones closer to 80 because it is the same type of ore that Cobalt was to Wrath of the Lich King, meaning it’s the first ore you’ll run across when entering Cataclysm.
The best spot and only spot that is recommended to farm Obsidium Deposits is Mount Hyjal because it is accessible via flying and there are no pre-quests to get to this zone. The other option is Vashj’ir which is an underwater zone that was added in Cataclysm. Most players do not prefer to mine under the water and that is why Mount Hyjal is currently the best zone to farm it in.
Keep in mind as you do quests through Mount Hyjal the zone will change in some areas, this should not have a major impact on the mining routes in Mount Hyjal but the new phased in content idea of this expansion is in full effect.