In Path of Exile, many items contain colored gem sockets. Skill gems of the matching color may. I this Path of Exile guide I go over some of the finer points of crafting sockets on gear. If you have any more. What the others above me said AND item level (not the item lvl requirement) plays an important factor. If an item doesn't meet a certain ilvl.
Path of Exile Linking and Gems
Poe gem slots - Hotel