What are gem forges used for?

Solution 1:

You can make only make square or circle gems go above +400 of their given stat at a gem forge by combining 3 similar gems that are all +400(stat).

There are also certain rare gems that can be crafted at the gem forge.

I'm trying to find info on this but I think if all combined gems are the same shape, the resulting gem is also that shape. Similarly, if all gems give the same stat then the resulting gem is a stronger version of that stat also.