Questions about gems rarely seen in build

Why not using a pain enhancer with firebird?
Also could invigatoring gemstone be a good gems in any build as it’s healing could be good?
Mutilation guard could it be good for monk or barb?
Is Mirinae only good for crusader?
And how Molten is not more use as it seems useful?

The main reason is that you’re limited to 3 gems, where in reality most of the time you only have 1 slot with some level of freedom. What’s normally used is simply superior to other gems. Bane of the Stricken is mandatory for most builds when you’re pushing, Bane of the Trapped is arguably the best gem in the game because of its damage increase and the fact that it enables itself with the rank 25 bonus. The last slot is usually where you have some freedom, although ultimately you use something that enhances your build: Zei’s (if you’re ranged), Enforcer (if you’re running a pets build), Esoteric Alteration (early on if toughness is an issue), Gogok (if your build requires CDR, attack speed bonus also helps if you use Zodiac), Legacy of Dreams (for obvious reasons), Simplicity’s Strength (buffs Primary Skills), Taeguk (for channeling builds), Bane of the Powerful (speedruns mostly), or Pain Enhancer (for AS oriented builds).

In some cases you don’t need Stricken until a very high push (Crusader AoV), or there’s simply no room for Trapped (various LoD builds).

There are no viable builds for Wizard, especially with Firebirds, that utilize attack speed (with the exception of Vyr’s Chantodo, but even then, Pain Enhancer is pretty pointless because the damage increase from attack speed is capped within Chantodo’s). Pain Enhancer simply isn’t worth it over the gems I’ve listed above.

Not at the expense of the damage you’re missing out on from dropping one of the other gems. It’s used in some very, very rare cases though. Not for its healing bonus, but for its rank 25 effect which grants immunity to control impairing effects.

It’s not the worst gem, and I remember it being an option for early Star Pact builds that couldn’t use Illusory boots or Teleport. Other than that though, Esoteric Alteration is much more preferred as it offers damage reduction to everything but physical attacks, as well as increasing your resistances (not physical) by 75% when you’re under 50% health.

This gem is not viable for any class. Not even in the zDPS challenge rifts we do weekly. I actually had to look up what it did because it’s so forgettable.

Mostly because the shield scales of life regen, which means the total shield amount can never really be that high. I think some people have tried to use it in a setup to have Squirt’s uptime, but it simply isn’t viable enough. In groups, Monk will provide a shield with superior uptime, while Wizards have setups which can easily grant 1.2m+ HP shields which can be refreshed and kept up with Magic Weapon (Deflection) in combination with Ashnagarr’s Blood Bracers.

Once all the new sets are out, and some legendaries are reworked, I really hope Blizzard takes a look at some of these gems. They’re only used for augments at this point.

Mirinae once had a place in a necro Bonestorm build as the main damage source. The build performed surprisingly well at the time. Unfortunately the proc rate of the Bonestorm was gimped and the build pretty much died.

Cool! Didn’t know that. A google search shows it got nerfed back in 2017. I wonder if it’s possible to revert/further improve the old iteration, considering how far power has grown in the game since then. Although I hope they just rework the gem into something completely different.

Well it’s sad because the main goal in a game where you have this kind of options as gems is to try some combo. Blizzard I think failed this now. But thank you your your complete answer.