A single mage with 4 or more non-holy magic paths carrying 3 or more gems…
or
A single commander carrying five or more magic items…
or
At least 10 non-holy magic paths across any number of mages and at least one gem….
or
Total magic items x 3 + gems + non-holy magic paths is 16 or more and at least one non-holy path…
… always triggers gem use.
Note that blood slaves count for half when calculating total gems