I discovered this purely by accident, but helps out a lot. A perfect emerald gives +29 poison damage. Put that in a socketed weapon and you merely get that value, added by whatever you choose to put in the object. Instead, I took a 'Artisan's War Axe' with three sockets and an included magical property of something like +12 poison damage over 5 seconds. I put in three perfect emeralds and the axe now has +659-703 poison damage over 7 seconds.

Basically, find a socketed weapon with a magical property, whether it be fire, lightning, cold, poison. Fill the sockets with gems that match that property and increase the gem's property exponentially.