with my knowlage this far i can surely say this: u need max slots for a weapon to glow ( exception is a shield where u need only 3 slots ) u need any type of gem ( here its tricky . . . if u put all gems the same element u will get this element's glowing effect for instance if u put 4 poison gems u get green glow, 4 pyramids and Rhombuses u get nice Bluish glow . . . but u can make a glow effect with different elemental gems for instance 1 poison 1 lightning 1 fire and 1 ice in that case u get purple glow . . . quite a long bracket huh
) and lastly some weapons max slots aint 4 . . . daggers max is 2 1 handed knight weapons' is 3 which means u can get a glow with less gems . . . i hope that i helped