Corunis eggs are actually just natural ruby crystals that have been enchanted either by a powerful magic user or some other powerful magic force.
Coruni (plural) are a leftover from Ark's past. In times long past golems were created out of a variety of minerals and precious gems. Each different type of mineral or gemstone was crafted into a different species, with each one having a different function. One of these was the Corunis. Crafted only from high-quality rubies, Coruni were created for nobility as show pieces.
A side effect of the golem creation process was a slight magical aura. Minerals/gems of the same type as the golem would be attracted to the golem, like iron to a magnet. Over time the aura becomes stronger, potentially imbuing the golem with magic abilities of its own. It is believed that this is how the Corunis survives to this day despite the fact that the knowledge of how to create golems has long be lost. The oldest living Coruni could enchant rubies that become attracted to their auras, thus continuing the species.