i'm using cutlass with +12 attack damage. +20% from 4 diamonds make that 14,4 attack damage. alternatively i could use 2 quartz blocks that would make that 14 damage. so from my understanding diamonds are better.
does anyone know if diamonds affect damage from fire aspect, smite etc.?
The %damage only affect your base attack
It applies to the attack before smite/bane/sharpness
In your case, it might still be a good idea to use diamonds, but on a fresh armor, quartz will give you more