Inquiry
What is the origin of the Rainbow Shell?
Theories
Nautilus Compression
JustinS1985
Huge Nautiliuses existed in prehistory, though the Ice Age resulted in their extinction. Nonetheless, a certain shell was held by the Reptites in the Tyrano Lair. When Lavos impacted the planet, the high pressure and temperature of his contact compressed and enriched the shell.
Magic Material
Doctor Shaft
The Rainbow Shell is regular material imbued with concentrated magic. When Lavos impacted the planet, Lavos emanated magic energy while spinning from his meteoric trek; this ground the material into a spiral shape and also gave it magic. The Shiny material of Chrono Cross could be considered similar to this effect.
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Inquiry
Why is there one Rainbow Shell in Chrono Trigger, yet many fragments in Chrono Cross?
Theories
Shell Split
ZeaLitY
After Porre conquered Guardia, they came into posession of the Rainbow Shell. Given the militaristic state of the world, it befell some accidental fate, possibly involving an explosion, that fragmented it into several pieces. Through the Acacia Dragoons, merchants, and Porre's vested interest in El Nido, these pieces made their way to the islands.
Dragonian Consistency
Legend of the Past
Since Dinopolis is from the Reptite Dimension, perhaps the Rainbow Shell from prehistory in that dimension came to be stored in the Dragonian metropolis. When Chronopolis bombed Dinopolis during the war in El Nido, that Rainbow Shell was fragmented into several pieces around El Nido.