Where can I find a diamond? (Risen 3: Titan Lords)

On Tanaris, near the lands of shadows, there is a half-dead Shaxx, help him with the quest, kill the local lord of shadows, then take the package to his sister in the city of magicians (don’t give it to anyone except your sister), return to him and he will give 2 diamonds.

on takarigua, go through the treasure hunter's quest - they will give you a diamond as a reward

There is a merchant on Kiel in a wrecked ship (where pirates hang out), I don't remember his name, you can buy 2 diamonds from him

EndorphiN179
You can't buy a diamond on Keel in a wrecked ship!
Don't fool the people.