I just started playing this game 5 days ago and earlier this day I got my very first obsidian crystal… I’m confused because it said it adds powerful affixes but the things is I dont know how powerful can it add to my weapon or other equipments…
Crystal affixes. Look at the DQ wiki for a list of possible affixes. They’re essentially affixes that are 3x as “big” as epic affixes (epic +HP% is 25%, crystal +HP% is 75% etc.)
Here is a list of all of the affixes you can get from obsidian 
Yup, obsidians provide affixes which are 3x better than epic affixes. And they convert your item to a Crystal Legend with a blue colored affix 