Thanks all for your answers. I also made this Question to a Youtube channel that makes builds for the game and this is his answer.
My Question:
Let’s say my daggers have + 20% lightning damage what is the best gem to add on it? Add a lightning gem or add the spike gem to boost the physical damage of the weapon. My playstyle is a mix of normal attacks and focus attacks.
His answer:
The spike will increase your basic attack damage as well as your rune damage. If you use a lightning gem it will only give damage to your basic attack NOT the rune skills. So my recommendation is to go with the damage increase gem, the spike one.