this post explains what can be used to Craft an Eternal Item. only Calcite can be used on an Eternal Pet. if you look in the Codex, there are a few Items that say you get a random affix instead of a certain affix. there are only 2 Eternal items that you can add affixes to, but then you can’t change them after they are added. they are also the only 2 Eternal items that you can put a Mythic affix on.
this is why it is easy to tell if someone has cheated on their Build or not. I don’t worry if a player has perfect values on all their affixes on their Eternal Items. maybe they spent 10,000 Dust to get a perfect Eternal. but if the Eternal Item has an affix it shouldn’t, like Block on a Necklace, that is a Hack.