Unique-Equipped

From WoWWiki

If an item says it is "Unique-Equipped", it means that you can have only one of those items equipped at any one time. You can have as many as you see fit in your inventory, but only one actually equipped to one of your items.

This is mostly to prevent absurd overpowering of characters with amazing gems, but still allows them to craft and sell them to others even when they already have one.