Secondary color gems will match either of the colors when socketed. For instance, an orange jewel may be used to activate a product bonus having a red or yellow socket.
Secondary color gems will count toward both of the colors when activating a meta jewel. For instance, a meta jewel that needs 5 red and 5 yellow gems could be activated with 5 orange gems.
You are able to replace a jewel already socketed within an item with a different one, destroying just the gems you're replacing.
A jewel doesn't have to complement its socket color to count toward a meta gem's bonus.
See also Edit
Help guide to finding gems around the ah - listing of the stats a named number of gems gives (including pre-WotLK gems)
Blacksmithing Sockets - for adding additional jewel sockets to products
Jewelcrafting - for skills needed to craft gems, training and patterns
Jewel Materials - for materials needed for any Jewelcrafter to produce a jewel
Mineral - for that raw supply of some the jewel materials
