Cinnabar Ore can be found at any level in the Overworld. It would require an iron pickaxe(or higher) to mine. It would also produce native cinnabar clusters if mined with the Pickaxe of the Core.
Cinnabar Ore can be smelted in a furnace to produce quicksilver. Native cinnabar clusters can also be smelted in a furnace to produce 2 quicksilvers.
(The correct amount is 2. The above was displayed incorrectly due to image problems.)