Some gamers in Diablo 3 forum think that legendary gems are too op. For this issue, the designer John yang replied in the post, discussing about the designing thoughts. Here is the original post:

First off, our philosophy on Legendary gems is that, when you pick one up for the first time, you should immediately want to use it (assuming it’s for your build – some gems obviously don’t apply to all builds/needs). This puts a minimum bar on the starting power level. After that, the power increase per upgrade is directly related to the frequency of upgrading, which is closely tied to the Greater Rift reward scheme and is still being iterated on. In the end, we’d like gems to be infinitely upgradeable when it makes sense, but not be outrageous.

Additionally, all Legendary gems have been disabled for followers. These powers were not designed for followers in mind, and this way we can ensure the gems are of a high power level because you can only have 3 equipped. In the future, if followers need avenues of advancement we would likely make Legendary gems specifically designed for them.

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