Does applying emerald to an item(specifically crystal leg) up to 50% makes the item eternal legend?
Emerald help improve item quality. Item quality cap at 25%
And item quality help improve getting perfect stats on gear while crafting/rerolling and I think a better chance of the stats you want
As VicBot said, item quality is capped at 25% for normal/legend/crystal items and 50% for eternals.
FYI, you cannot create eternals.