Basic Trope: An item that the story's players want to attain that doesn't actually do anything within the story, and could be replaced with a different item without altering the plot.
Straight: Alice and Bob are searching for The Magic Crystal Of Shadowland, and so is Emperor Evulz. It's never actually explained what the crystal does. In other words, the plot would be the same if the crystal were replaced with some other irrelevant Plot Device, such as a spellbook or a grilled cheese sandwich.
Alice and Bob are searching for an artifact that they don't even know the appearance or purpose of, just because someone told them it was important.
There are several MacGuffins which come up one by one.
Every character in the work wants the crystal, including very minor and/or unseen characters.
Downplayed: The Magic Crystal of Shadowland is show to have healing power, but so do the Crystal of Light and the Crystal of Nature.
Justified: The object in question is legendary for its power (or other reasons), explaining why everyone wants to get it no matter what it is.
Zig Zagged: The story concerns a "retrieval team" hunting after valuable artifacts. Sometimes the artifacts have useful properties that the team itself needs, and sometimes they're hunting after items of seemingly little value that their employers nontheless want.
The Plot Device used in the story is actually relevant to what the heroes need.
Defied: Diane tells Alice and Bob, "The Magic Crystal of Shadowland is useless against Emperor Evulz; Charles must have lied to you."
Discussed: "Why do we actually chase after this thing? Is it really that important to us, or to anyone?" "I'm sure it must have some use in the end, if all the boffins say it has one."
Conversed: "It doesn't make any sense for Alice and Bob from that show to chase that magical crystal or whatever. They don't even know what it does!"
Played For Laughs: Alice, Bob, and the others know that the MacGuffin is a smooth stone somewhere in the river. However, they cannot agree on which stone it is, leading to a hilarious argument. If it's just Alice and Bob, a Duck Season, Rabbit Season routine is likely.
Played For Drama: Alice and Bob kill Charles and David over the MacGuffin.
Untwisted: At first, Alice and Bob think that the Magic Crystal of Shadowland will help them defeat Emperor Evulz. Even after they find out that the crystal is useless to them, they still want it for some unknown reason.