There are 5 types of Ore that can be mined for money, that is, the Blue Ore, Red Ore, Yellow Ore, Green Ore, and Black Ore. These five are considered as normal ores which can be recycled by the Ore Processing NPC. Refining the ore can sometimes produce another ore or bonus items (recovery items, talics, gems).
There is a special ore that can not be sold to NPCs, this is called "Rare Ore". This kind of ore can only be refined in certain stages by the Ore Processing NPC, which can then be traded for a box at the Gem Collector NPC. If the box is opened, a random type of mining ore (+1, +2, +3, and sometimes even +4) level ore can be obtained along with other possible items (equipment, talics, etc).
Since the Episode 2 update, ore has no value when sold to vendors. In order to profit, it has to be purchased by other players on the Auction House. The ore may be processed, but the materials when sold off to vendors usually does not meet the cost of processing the ore. Other items such as gems or talics can be sold in the Auction House (AH) for increased profits.