Can be upgraded up to level 100.
You can mine gems in the gem mine. Mining a gem costs resources and takes a certain amount of time.
Mining gems is worthwhile because sometimes workers also find Shadow World keys or pet eggs. Everything that is found ends up directly in the backpack.
Once you or someone else from your guild has upgraded the Hall of Knights to at least level 15, rare black gems can be found that give a bonus to all attributes.
During the Black Friday event, you will only find rare black gems in the mine regardless of your Hall of Knights.
The gem mine can only be upgraded once the main building has been upgraded to level 3. From level 25 it is possible to mine gems while upgrading the mine.