https://e621.net/posts/3433160 I am fairly certain that we count front legs as "arms" for tagging purposes on quadrupeds, but can't remember where I saw that or if I imagined it. Also, is it technically leg wear only?

Head_gem tag is criminally underused since I'm fairly certain that there's thousands of examples. Pokemon alone has entire species that have them. I wonder - does Meowth's gold count as not a gem, even if a mineral, because not a crystal? Not sure which definition of "gem" is being used. Like, some very dull-looking stones can be 'gems' for some purposes. Think: Hematite which is dark gray

It's possible that there's a variant of on head_gems that is more used, but I haven't found it, yet. There's 22 or so examples tagged, so best to look into this before there's tons. Tempted to do a Wiki entry and link to it from the appropriate anatomy entry.

Thanks for help! :)

I think forehead_gem is probably what you're looking for. (Although I agree it's undertagged.)
Most of head_gem look like misremembers of forehead_gem. Except for post #3416797 which might actually belong there.

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I don't know what the current or official stance on tagging the forelimbs of quadrupeds is. But I think it often falls into a bit of a grey area depending on how they're drawn per image, orientation, etc.

In general though, it often boils down to this:
- If they're using them as arms then use arm-related tags for them. They are often drawn as if the forelegs are analogous to arms, so treating them like that is often what makes the most sense for their position/gestures/etc within an image. So if they're doing non-leg things with their forelimbs, or the forelimbs are bound more like how arms would be... then tag it accordingly (as being an arm thing).
- But if they're currently standing on them like legs and/or treating them exclusively like actual legs, then use leg-related tags for them. Because in those cases that is what makes the most sense in that picture.

It's not perfect, but it makes a pretty decent rule of thumb for it.

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[Also, this might be the discussion you were thinking of: https://e621.net/forum_topics/26716 ]