They are used to hatch Chameleon pets which require level 90 Summoning.
To hatch the egg, it must be taken to a pet shop and placed in the incubator at the back of the shop. The egg will hatch into a Chameleon after an hour of gameplay.
If you try to take it if you do not have the summoning requirements, it will say: "You do not have a high-enough Summoning level to take care of that pet." Or if you cast Telekinetic Grab on it, it will say: "These eggs are too delicate to take with a spell."