Problems with a gem

The gem called "Item rarity" is working?
I have put him on a slot and open the Character stats screen but dont give me any percentage in "Increased rarity of itens dropped by slain monster ". Rings, helmet and colar with this ability change the stats but the gem not, supposedly would give me 30% of more chance.

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In addition to active skill gems, another type of gem exists – support gems. These are placed in special, linked sockets and grant additional properties to other gems they are linked with. They do not grant active skills themselves.

It means its a support gem. It will not give you 30% to rarity by itself. It must be linked with skill gem to which it will grant additional 30% to rarity if you kill monsters with the skill to which this support gem is linked. To be able to see how much item rarity increase you have you need to click on the skill icone to which your support gem linked on the character info pannel.

Thanks! this clears the things to me