Rainbow Shell Sidequest - Starts in Choras Village 600 AD
When speaking to the Old Man in The End of Time, the sixth side quest that he tells you about is to acquire the Rainbow Shell, an item that's also used in the Sun Stone Sidequest for Crono's best weapon. The quest's flavor text is, "There's an object in the middle ages that sparkles like a rainbow."
To get started with this side quest you'll want to head to Choras Village in 600 AD. Speak with Toma in the Cafe and he'll tell you that he has a lead on the Rainbow Shell but if he doesn't return he wants you to pour Toma's Pop on his gravestone. Get on the Epoch and travel to 1000 AD and return to Choras Village, you'll notice that northwest of the village there's a gravestone on the world map called West Cape; approach it and enter.
It's a small area and at the highest point you'll find Toma's Gravestone. Examine it and a scene will commence where Crono pours Toma's Pop onto the Gravestone, the spirit of Toma will come out and tell you about a location, Giant's Claw, which is found in 600 AD so return there after the scene. Before leaving there's also a Speed Tab to find behind Toma's Gravestone!
If you didn't pay attention to the cutscene where Toma shows you the exact location, use the screen shot above that's on the right - that's the entrance of Giant's Claw. This is a pretty long but rather straight forward dungeon. When you reach the bottom of it you'll find the Rainbow Shell which is shown in my screen shot below. We're not done yet though!
After finding the Rainbow Shell you'll get caught up in multiple scenes that take place in Guardia Castle. I won't go into too much detail about what's going on but basically Marle's dad is suspected of a crime that he didn't commit and the Chancellor guy is off his rocker.
You'll have to battle it out with the Chancellor in the court room - it's a tougher fight than the previous dungeon but not too bad over all assuming you've got some levels on your characters. After you beat up this boss more scenes will commence.
When you gain control of your character run down to the Rainbow Shell again and you'll find Melchoir. Speak with him and he will craft either 1 Prism Dress for you or 3 Prism Helms. A Prism Dress is the best in slot chest armor for a female character and the Prism Helms are decent helms. I highly recommend you go with the Prism Dress and equip it to either Marle or Ayla.
After you complete both the Rainbow Shell and Sun Stone Sidequest you'll be able to have Melchoir craft you Crono's Ultimate Weapon, Rainbow.