Save gems for the only building that requires gems: builder huts. If you’re at five builder huts already (the maximum number allowed), then spend gems to complete upgrades or build units faster. Never use gems to buy gold or elixir.
Mobile Gaming eGuide - Tips, Tricks, and Strategies for Today's Hottest Mobile Games Clash of Clans
Save gems for the only building that requires gems: builder huts. If you’re at five builder huts already (the maximum number allowed), then spend gems to complete upgrades or build units faster. Never use gems to buy gold or elixir.