Using the crystal Amethyst will give you a random list of set(green) affix to choose from. But there are some affixes that cannot be obtained from Amethyst and can only be obtained through drops. An example would be an item that you already have, the Stone of Nadroji ring.
These are the affixes that cannot be obtained through Amethyst:
- Crystalline
- Mythical
- Nadroji
- Eternalized
- Defiant
- Maelstrom
- Gladiator
- Seven Deadly Sins
- Masochism
- Smoke Screen
- Cerebral Vortex
- Mayhem
- Epiphany
- Berserker
Credits to Crystal Crafting Restrictions for the list.
Read more about crystals on that thread (Crystal Crafting Restrictions) and this: Griffin's Smithery: Crystals