A repost of a theory from reddit relating the Happiness Stone and the Divine Zero Orb. You can read the original theory here https://www.reddit.com/r/Falcom/comments/17nqpgn/moon_door_5_theory_the_happiness_stone_is_the/.
TLDR: The happiness stone has a lot of similarities to the divine zero orb which suggests they could share the same purpose of storing Sept-Terrion memories so that they can fulfill their purpose without morality or pain. Note that I have not played the cold steel ark.
The happiness stone is one of the most mysterious objects in the trails series and the confusing door it’s paired with is not very helpful (if you want a synopsis click this link). My theory is that it is the cast aside memories of the Sept-Terrion of space, the Aureole, and to prove that we need to start with another Sept-Terrion, KeA–the Sept-Terrion of zero.
If you maxed KeA’s bonding points and did the quest involving the whitestone in the intermission, then during the final chapter of Azure, you will get a powerful accessory, the Divine Zero Orb. As shown in picture, the orb glows a bright silver light. This can be just a gameplay element to show the player that there is something on the table if not for the fact that the orb’s item description mentions it glowing. Tio mentions that the orb contains KeA’s lingering emotions–that she left her sorrow and indecisiveness in the stone so that she will be able to perform her duty.
I believe that the same thing happened to the Aureole. The Aureole main purpose was to make humanity dreams true (or maybe it was the purpose the people of Liberl Ark gave it). In the Lakeside Underground Facility (link) Celestin states that “Shortly after the Aureole gained autonomy, it turned to simply one-sidedly bestowing its gifts regardless of the wishes of the people.” Meaning that the Aureole wasn’t bound by morality and emotion (she uses the word autonomy a lot I think it’s not autonomy in the sense of a free ability to act but in the sense of having no mortal bounds).
The happiness stone has a golden glow and seemly transports the girl to the past, from data mining we can see the dragon is named “young Ragnar” meaning it’s the same dragon we see in the game (and the defender of the Aureole) both of those thing tie the happiness stone to the Aureole. I think that the stone didn’t transport the girls into the past but let her see the memory within (the same thing that happens when touching the divine orb) which explains all the time travel shenanigans.