Most of the "buff" provided by the gems are linked to one and only item's entry.
You will most likely copy/past a new entry in the dbc named SpellItemEnchantment.dbc and in GemProperties.dbc.
It's easy to understand once you get this here's an exemple :
I've created a gem (prismatic one, meaning it goes in red, blue or yellow) wich give +100 to spirit.
here's a screenshot of the SpellItemEnchantment.dbc; the 300145 is the ID of my Item used to be the gem in game, let's call it "the caching entry". I don't know why but the facts is when you didn't add it, it's not rendering correctly.
For the full descriptions about the collums go at
https://wowdev.wiki/DB/SpellItemEnchantment
then you can see my GemProperties.dbc as you probably though 4030 is the same as my SpellItemEnchantment's row ID.
For the full descriptions about the collums go at
https://wowdev.wiki/DB/GemProperties (Keep in mind that the wiki say "Gem Color (a combination of Meta = 1, Red = 2, Yellow = 4 and Blue = 8 )" but you can add "14" as the prismatic one.)
Once it's done you just need to create a new gem with the exact same ID as the caching entry in SpellItemEnchantment, change it's gemproperties with the one you've just created and of course, upload the dbc on your server before restarting it.
Hope it helps.
