I would start with a Jewel that has both Crit Damage and Spell Crit Damage and then try and hit the CDR with the Desicrate personally. Jewel crafting in 5.5 is insanely expensive so I usually don’t Omen of Light and just sell the Jewel to find the next one until I get the mods I want.

Diamonds are really tough cause of how many mods they can roll. I’m not exactly sure how the math works out for them, cause you can sink hundreds of chaos orbs into them before you get your 2nd mod, then you risk the +1 suffix, so I think omens of light are probably cheaper overall. Tough craft, the sort of thing you probably start with a crit fracture instead, and sell anything along the way until you finally hit cooldown on the desecrate.