Should the child with the Heart'S Egg (Shugo Chara!) become burdened by worries and doubts, the Heart's Egg will slowly become tainted black. If the child gives up his or her dreams instead of pursuing them, it will eventually become evil and mischievous X-Egg and attack everyone in sight when it hatches into an X-Character.
It represents a typical Egg in shape, but is purely black and has a large white "X" over it.