You may be looking for the artbook The Heart's Egg...
As the child becomes an adult, the egg will disappear without a trace. However, it can be assumed that if the child stays attached to his or her hopes and dreams, the egg will stay even through adulthood.
It's revealed at the end of the manga that the Heart's Egg is actually the Embryo.
List of Guardian Eggs
Ran's egg is red with a plaid pattern. A black stripe aligned with hearts on the edges cover the center of the egg, and in the stripe are red hearts.
Miki's egg is blue with a plaid pattern. A black stripe aligned with circles on the edges cover the center of the egg, and in the stripe are blue spades.
Suu's egg is green with a plaid pattern. A black stripe aligned with crosses on the edges cover the center of the egg, and in the stripe are green clubs/clovers.
Dia's egg is yellow with a plaid pattern. A black stripe aligned with flowers on the edges cover the center of the egg, and in the stripe are yellow diamonds.
Il's egg is purple on one half of the egg, black on the other. A black, Gothic lace pattern adorns the center of the egg.
El's egg is pink on one half of the egg, light pink on the other. A light pink lace pattern adorns the center of the egg.
Yoru's egg is black with a white zigzag stripe across the center of the egg. In the stripe are cat's paw imprints.
Kiseki's egg is baby blue with a yellow crown and two swervy, yellow lines going across from the crown.
Temari's egg is light pink with pink flowers on the egg.
Rhythm's egg is dark blue with blue flowers on the egg.
Daichi's egg is blue with an orange circle on the bottom and a yellow star in the circle.
Pepe's egg is pink with a magenta, cloud-like stripe going across the center of the egg. In the stripe are cream rabbit head imprints.
Kusukusu's egg is checkered with red and white diamonds. A mauve stripe goes across the egg, and there is a mint green star and two light blue teardrops in the stripe.
Musashi's egg is light green with a bamboo stick imprint in the center of the egg.
Nana's egg is mauve with a multicolor flower brooch in the center of the egg.
Hikaru's Guardian Egg is yellow with little gold stars decorating it.
Hotaru's egg is yellow with an orange sun in the center.
X-EggsAlso known as Batsu tamago (Punishment Egg).
X-Eggs are known to say muri which means impossible or useless.
Should the child with the Heart's Egg become burdened by worries and doubts, the Heart's Egg will slowly become tainted with a black color. If the child gives up his or her dreams instead of pursuing them, it will eventually become an evil and mischievous X-Egg and attack everyone in sight when it hatches into an X-Character.
Some X-Eggs however are quite friendly such as the ones that stayed at Rikka's apartment building. They cared about Rikka and explained "how they had a lot of fun with her" after they were purified by Pure Feeling and Amulet Diamond.
X-Eggs can also build up a lot of stress and eventually self destructs due to its extreme negative emotions. Not only that, but when enough X-eggs are around in an area, they can perform formations that can be quite challenging to win against them, or they can combine powers to create a "bigger X-Egg" and hatch to become a "Mega X-Character" which can be EXTREMELY dangerous.
Only Amu Hinamori, Utau Hoshina, and Rikka Hiiragi have been able to purify the X-Eggs, along with an attack Yamato Maihime and Clown Drop can use together called 'Queen's Waltz'. Platinum Royale and Amulet Heart can also use 'Platinum Heart' to do the same.
List of Victims
- Suzuki Seiichiro (Episode 2)
- Yuki Hatoba (Episode 4)
- Yamada (Episode 5)
- Maika Himekawa (Episode 6)
- Misaki Watarai (Episode 8)
- Mifuyu Torii (Episode 14-15)
- Takuya Nakagura (Episode 16)
- Unknown victims (Include Sion) (Episode 20)
- Unkniwn victims (Episode 23)
- Amu Hinamori (Episode 27, not immediately, but Dia)
- Unknown 2 victim (Episode 30)
- Shouta (Episode 34)
- Shuu Hinamori (Episode 35)
- Saaya Yamabuki (Episode 37)
- Unknown victims (Episode 37)
- Unknown victims (Episode 40)
- Unknown victims (Episode 41)
- Marimo Hatanaka (Episode 45)
- Kotone (Episode 49)
- Unknown (Episode 50)
- Unknown victims (Episode 51)
- Hikaru Ichinomiya (Episode 101 & 102)
- Unknown victims (Episode 102)
- Unknown Victim (Episode 104)
- Yuka (Episode 115)
- Unknown victim (Episode 116)
- Unknown victims (Episode 121)
- Rikka Hiiragi (Episode 122, not immediately, but Hotaru)
Reasons of Heart Egg Becoming an X-Egg
Mystery EggsIn the second season of the anime, the new antagonist Lulu can use a ruby necklace and Character Change to corrupt her target and turn her egg into a Wishing Egg, also known as "?-Egg" or "Wishing Egg", a ruby-colored egg with a question mark on it.
List of victims
- Manami (Episode 54)
- Hitomi Ninagawa (Episode 55)
- Mamoru Mizuno (Episode 56)
- Yugaku Iwagaki (Episode 57)
- Koyomi Hirano (Episode 60)
- Santa Claus (Episode 63)
- You Ebihara (Episode 64)
- Kiko Ebihara (Episode 64)
- Chiyoko Nakayama (Episode 66)
- Nayuta Kusanagi (Episode 67)
- Saaya Yamabuki (Episode 68)
- Fuyuki Kirishima (Episode 69)
- Yukina Kogure (Episode 70)
- Young Girl (Episode 72)
- Mimori (Episode 73)
- Haruki Maruyama (Episode 75)
- Nami (Episode 78)
- Nemi (Episode 78)
- Natsuko Nishino (Episode 82)
- Young Girl 2 (Episode 83)
- Wakana (Episode 84)
- Yua Sakurai (Episode 86)
- Lulu De Morcerf (Episode 89)