At Rubik’s Crystallography guide he said:
So, he take an item with anyother affix to put Obsidian and hope that the one he want appears.
Considering that you can’t have the same affix twice even if it have diferent quality (Ex: Can’t have Legendary +HP% and Crystal +HP% at the same time), before you use the Obsidian, spend some Topaz/Angelite to add affix that you do not want to be crystal (Ex: You want Crystal +225% Luck, add rare/epic Critical Chance, Critical DMG, HP%, Flat Armor and Flat All Resist on the other five slots, and keep just the space to the Obsidian), just then you add the Obsidian. So, instead of having 4/20 (4 Crystal Options to choose from out of 20 Crystal Affixes) chance, you would have 4/15(4 Crystal Options to choose from ou of 20 - the 5 you already have). At cost of just some angelites.
Edit1: The 5 Normal/Epic affix you’ll add don’t need to be random, add the 5 that would be worse to pick. (Ex: If you prettend to add +Crit DMG to Helm and +Crit Chance to OH, don’t add Normal/Epic Crit Chance to Helm, cause if it comes Crystal, wouldn’t be a problem, u’ll just have to change plans instead of waste an Obsidian).
Edit2: Saw that on Griffins’ crystal topic:
So wouldn’t be 4/20(20%) and 4/15(27%) but 5/20(25%) and 4/15. Lesser help in my tip, but still a gain. Adds some chance to have the one you want while still helping to exclude the worse ones.
(Considering that you aren’t hacking like everyone else, ¬¬)