There are many sources.
You can read the mind of Charlie the Goblin by the Yggdrasil quest.
In Hohenzell (dunno the english name, the village with the mines) you get 2 (4 with mindreading) malachites.
The black ring vendor that fears you, in the fortress where you disable the 3 anti-dragon-zones, sells you malachites
and many more sources

He, who does not fear his own sword, is not worthy to hold it.