Eternal Items can have Crystal Affixes, but they have to have an affix that is on the Crystal Affix list to have a chance for the affix to be Crystal. you can get an Eternal Item with a Crystal Affix either as a Loot drop or buying one from the LegendEx. a Farming Build with lots of Luck & Crystalline Set can improve your chances.
here is a post on which Crystals & Myth Stones can be used to Craft Eternal Items.