It consists of a starry sky overlooking a citizen and the Queen of Ambrosia beside the sea.
Hidden in the seal are three harmonious songs; A Song of the Stars, A Song of the Sea, and A Song of the Sun. The star-filled sky is the treble clef piano sheet music for A Song of the Stars, the symbols surrounding the citizen and the queen are coded notes for A Song of the Sea, and the sky again (turned upside down) is the base clef sheet music for A Song of the Sun.
Jean Descole managed to decrypt A Song of the Stars and A Song of the Sea, but before he could attempt to raise Ambrosia, Melina Whistler, the only one who could sing the latter, died. Deceiving Melina's father, Oswald, he had the Detragan constructed in hopes of resurrecting her by copying her memories into another girl's brain. Most of the subjects rejected the process, bar one.
In Dr. Schrader's flat, Emmy found a piece of rock with part of the Ambrosian Seal carved into it. She took a photo of it while Dr. Schrader told her his knowledge of the city.
When the 'surviving' contestants arrived at an island, Marco Brock pointed out the Seal of Ambrosia carved into a huge rock at the beach, successfully identifying the island as where the kingdom of Ambrosia used to be.
Janice was revealed in Eternal Diva to be Melina by Professor Layton. Upon this, Descole kidnapped her, explaining about the songs hidden in the seal and their power.
He then played A Song of the Stars on the Detragan's organ with Melina's singing, but failed to make Ambrosia rise again. Enraged, the Detragan transformed into the Detra-Gigant (a destructive machine) to force Ambrosia to appear.
Layton eventually figured out the hidden third melody by looking at the seal upside down. Playing this with A Song of the Stars and combined with Melina's singing made the ruins of Ambrosia rise out of the sea.