Can you level up skills from unique items with skill gems or they stay the level they droped?

Can you level up skills from unique items with skill gems or they stay the level they dropped?

Like Corpsewade Iron Greaves boots Grants Skill: Level 4 Decompose - can I lelev this skill? In the trade, I see those boots with the skills level from level 4 to level 18 and i was wondering if people are leveling it with skill gems or the boots drop a random level/or scale with player level?