Weapon levels for gaining gem slots

[Blackshirts]

You gain 1 gem slot for each weapon at levels 4, 10 and 18 so far. It appears there is one more slot but I have not reached that level yet(currently sword at 22 and other 3 at 20).

[Blackshirts]

Level 4, 10, 18 and 28 for each weapon to max out gem slots.