The item ID for Rainbow Shell in Stardew Valley is:
The above cheat code applies to all platforms including PC/Mac (Steam), XBOX One, PS4 and the Nintendo Switch.
Spawning items in Stardew Valley is not done via commands, but rather through a cheat in which you must set your character's name, or the name of an animal, to the item code of the item you wish to spawn surrounded by [ and ].
To spawn this item using an animal's name, visit Marnie's Ranch, speak to Marnie and purchase an animal (we recommend a chicken as it is cheapest). Name the animal  and press OK. You will then receive the rainbow shell item.
You can only set your character's name once, at the start of the game (when you create your world), for this reason we recommend you use the animal name method. If you do wish to use this method, first set your character's name to  when creating your save. Whenever you speak to a villager that says your name in dialogue (for example Gus at the Stardrop Saloon) you will be given the rainbow shell item.
If you need more help with spawning rainbow shell, we have a detailed guide on our blog - click here to visit that page.
The Rainbow Shell is an item that's required for The Mysterious Qi quest. You can obtain the Rainbow Shell at the Traveling Cart shop, and during summer by foraging at the beach. It can be sold for up to 600 gold. It has no known uses outside of The Mysterious Qi quest.
It's a very beautiful shell.