Gem Level Up button becomes grey

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Melvar wrote:

Possibly this is the fix to the issue where leveling up gems while in use could cause them to stop doing damage?

Yep, that's accurate.

There was a previously reported issue where leveling up Vaal Cyclone while you were channeling it would cause it to stop dealing damage (for the duration of the current channel). We investigated that report and made some changes to fix this problem. As a result of these fixes, you can no longer level up a gem while you're channeling or using a skill; this includes Support Gems linked to a skill as well. This is why you're seeing gems "grey out" when you use them sometimes, as it's very briefly preventing you from leveling them up.