Hi,
There are no mana gems in Classic Diablo II… so I suspect you are posting on the wrong forum.
If this is for World of Warcraft or WoW Classic… they have their own dedicated forums, here:
https://us.forums.blizzard.com/en/wow/
If it’s for another game, please refer to Welcome to the Classic Games Tech Support Forums - Please Read! for a list of Blizzard games and links to appropriate game forums.
Good luck !