In the changelog, it said that amethyst buds can be silk touched at any stage. However, when an amethyst bud that hasn't grown into an amethyst cluster is silk touched, nothing is dropped. 

Steps to recreate:

1. Go to an amethyst geode
2. Have a pickaxe with the silk touch enchantment
3. Mine an amethyst bud that is not fully grown with the pickaxe in survival mode
4. Watch it not drop anything