The most-used gems for Demonology Warlock 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 Demonology Warlock players enchant (with how many do). Don't leave them empty — missing enchants are the most common, cheapest performance gap. Open the Demonology BiS page for the exact enchant per slot, or paste your character into the analyzer to see what you're missing.