Alkahest is one of the two items required to enchant armor and weapons. The tier of the item to be enchanted determines the minimum tier of alkahest that is required. (Tier 3 equipment requires Tier 3 Alkahest.) Some alkahest will not work for all equipment even if it is of the correct tier. For example, Refined Godly Alkahest only works on Avatar weapons. Feedstock is the other required enchanting material.
With new level 65 gear system (Guardian gear) feedstock and alkahest is no longer used to enchant level 65 gear. It remains in use for leveling gear. Some types of alkahest are removed from game. Previously Masterwork Alkahest was the most important resource for enchanting level 65 gear from +9 to +12/+15.
The different types of alkahest are generally based on the the level of enchantment and the tier of equipment that can be enchanted. The list of the different types, the upper tier limitation, and maximum enchantment bonus is given below.
- Alkahest / Tier 5 Equipment / +6 Enchantment
- Godly Alkahest / Tier 3 Equipment / +6 Enchantment
- Refined Godly Alkahest / Tier 3 Avatar Weapons / +7 to +9 Enchantment
- Tier 3 Alkahest / Tier 3 Equipment / +9 Enchantment
- Tier 4 Alkahest / Tier 4 Equipment / +9 Enchantment
- Tier 5 Alkahest / Tier 5 Equipment / +9 Enchantment
- Tier 6 Alkahest / Tier 6 Equipment / +9 Enchantment
- Masterwork Alkahest / ANY Tier Equipment / +10 to +15 Enchantment
- Premium Alkahest / ANY Tier Equipment / +9 to +15 Enchantment
- Extensive Alkahest / ANY Tier Equipment / ANY Level of Enchantment
Wikipedia has an interesting article on Alkahast that can be found at: https://en.wikipedia.org/wiki/Alkahest