The most-used gems for Preservation Evoker in The War Within, plus which slots to enchant — aggregated from real Mythic raid and Mythic+ logs on WarcraftLogs.
These are the slots the top Preservation Evoker players enchant (with how many do). Don't leave them empty — missing enchants are the most common, cheapest performance gap. Open the Preservation BiS page for the exact enchant per slot, or paste your character into the analyzer to see what you're missing.