I've mentioned the Pearls of Pingaree from Rinkitink in Oz before, but I think there's a bit more to say about them. There are three of them, a blue one that grants incredible strength, a pink one that protects from danger, and a white one that gives advice.
They are said to have been a gift to King Kitticut's ancestor by the Queen of the Mermaids. Exactly what the ancestor did to earn such a gift isn't stated, but I remember reading a short story (by Fred Otto, I believe) that had Prince Brawn of Pingaree killing a sea devil. Makes sense, but I don't think the story was ever even published. In some ways, the Pink Pearl is similar to the protective circles that the mermaids make in The Sea Fairies, but it's actually more effective. While the fairy circles stop anyone or anything hazardous from touching an enchanted person, the sea devils are still able to capture Queen Aquareine and her companions by totally surrounding them. On the other hand, when the Nome King tries to trap King Rinkitink in a magical net, the power of the Pink Pearl enables him to walk right out of it. Similarly, he remains suspended above the bottomless pit that the Metal Monarch opens under him.
I don't know that the fairy circles would have been able to protect someone against such things. Also, the Pink and Blue Pearls transfer their power to anyone touching the bearer. And the White Pearl apparently has other powers beyond simply advising, as it supplies a boat that Prince Inga uses to sail to Regos and Coregos and free his people. Of course, magic like that seems like it would just make things too easy, and it kind of does.
In order to compensate somewhat for this incredible power, the story has Rinkitink, oblivious to their powers, accidentally throwing out the shoes that contain the pearls. Also, in the Nome Kingdom, there's a point at which Inga and Rinkitink split up and divide the pearls between them, so that the prince has the strength and the king the protection.
I'd say it's a credit to L. Frank Baum that he managed to make the story engaging even when the hero has almost limitless power. It makes it all the more disappointing that Inga isn't able to save his parents in the end, even with that power. It's likely that he DID have Inga succeed in his original King Rinkitink manuscript, but in making it into an Oz book he had Dorothy and the Wizard show up and bully the Nome King into letting everybody go. I'd like to know the original ending, and I'm not sure why Baum couldn't have had the people from Oz come onto the scene AFTER Inga had already won the day.
The idea of magical talismans that grant such powers was one Ruth Plumly Thompson used in Hungry Tiger, although here the items are only protective. The three Rash Rubies protect the ruler of Rash from danger on land, in the water, and in the air, respectively. I'm not sure why fire is left out of the picture, but the earth ruby protects against that as well, as seen with the fire fall.
Since the rubies will only work for the rightful ruler, Irasha throws them away when he steals the throne, and his nephew Evered has to find them. Like the Pink Pearl, they also protect anyone who's touching the ruler when he's carrying them. Evered does eventually find all three, which turn out to have been discovered by appropriate people. While the pearls were likely on Thompson's mind when she wrote this story, the tale itself is quite different from Rinkitink.