Can't apply gemstone to upgrade slots
One of my guildies had the same problem, after having him link everything I noticed that the item he was trying to put the gem into was level 10, while the gem was level 20. He got a pair of lvl 20 legs and the gem went right in. I believe the gem/rune level can’t be over the items actual level.
Fellstar is correct, the level of the item you wish to put the gem in must also match the level of the gem, not just you.
The Unnamed[ThUn] – Desolation
Vayra – Elementalist
Forkrul Assail – Mesmer
Vayra – Elementalist
Forkrul Assail – Mesmer