The cache of stolen jewelry in my recently completed (more-or-less) mystery, “Smoke,” is essentially a ‘McGuffin.’ That is, in and of themselves they have no role in the plot other than to be searched for. A box of chocolate bars would have worked every bit as well. Unless they melted, of course. In this, the jewelry is like the statue in “The Maltese Falcon.” It is an interesting bit of hardware but does not really have any influence on the plot. Its role is entirely passive — it exists only to be chased after.
On the other hand, the four jewels known as “The Eyes of the Wind” and “The Jewels of the Elements” in the two fantasy novels of those names, are completely tied into the plot. They are sought because of the power they can bestow on their possessor, and that power is used in the books. They also appear as an important element (but not driving the main plot) in “The Ways of Wizardry.” It is certain that they will reappear in that novel’s sequel(s).
To be sure, there is not a thing wrong with incorporating a McGuffin into ones work. It is quite common, in fact, and the basis for countless stories. But having an active object adds an extra layer of interest. It creates more incentive for those who seek it. A lost child or a time bomb needs to be found; a statue of a bird, not so much. That’s a matter of degree also. Some objects will be more critical than others. The fate of the world depends on who possesses the jewels of the elements.
But we can’t always be saving the world, can we? Sometimes, it will just be that valuable McGuffin someone has hidden away, and that is okay — as long as the story is told well.