The Rethban Ore (Item ID 2798) is part of the Quest Rethban Ore (Quest ID 347). It should be dropped by enemies within the Rethban Caverns, as well as Veins in the specific region.
On Zero, the Rethban Ore can be looted from all kind of Veins, since the loot templates aren't correct.
The gameobject templates 2054 and 2055 are the only ones which should have Rethban Ore in their loot template. This counts for Zero, One and Two as well. To fix the issue, use the following query to delete Rethban Ore from all loot templates it doesn't belong to.
FOR ZERO, ONE AND TWO:
DELETE FROM `gameobject_loot_template` WHERE `item` = 2798 AND `entry` NOT IN (2054, 2055);
While the gameobjects within the cavern seem to be fine on One and Two, the gameobjects for Zero are mostly a mess. The ones I found are listed down below.
ONLY FOR ZERO:
UPDATE `gameobject` -- Tin Vein SET `id` = 2054 WHERE `guid` IN (72064, 72065, 72066, 72082, 72083, 72085, 72086, 72087, 72123, 72124, 72125, 72126, 72127);
UPDATE `gameobject` -- Copper Vein SET `id` = 2055 WHERE `guid` IN (72000, 72001, 72002, 72009, 72010, 72011, 72012, 72051, 72052, 72053, 72062);
This way the Rethban Ore should only be mined in the Rethban Caverns.