Nope never found it, there are some other veins that gave me some malachite ore and other mixed ore in that cave. Maybe that is what the person was talking about.
I've seen this elusive malachite vein mentioned in a walkthrough as well but the person doing the walkthrough said they could not find it either.
Does not matter really as there are plenty of Malachite gems around I had 13 left in inventory when I finished the game. There's only a couple good enchantments that use them for level 10 enchantment and quite frankly the difference between level 9 that does not use them and level 10 that does is not much.
For how rare they supposedly are it should be a greater difference IMO.