Magic ring

The Magic Rings of Andrew Ketterley were a small collection of yellow and green finger rings, which he had created, to respectively transport people to and from the Wood between the Worlds.

Creation
The rings were created on Earth by Andrew Ketterley (in Earth-year 1900) from a magical dust that he had discovered in a box left to him by his godmother, Mrs. Lefay, who was part Fairy. She had made him promise her that he would destroy the box, only for him to instead end up keeping it, and spend years studying it, and trying to discover its secrets.

When he first opened it, he found some kind of dust inside, which he instinctually knew was magic. For even before he had opened the box, or knowing what was inside it, he had felt a tingling sensation of magic just from touching it. This feeling was likely emitted from the dust itself, and when Andrew did look inside, although it merely looked like sand, he knew immediately it was some higher force that he should avoid touching directly.

He finally found, by various magical tests and research, that the box had originated from the lost civilisation of Atlantis, and contained dust that had been brought from another world (although Mr. Ketterley did not know this at the time, the dust was from the Wood between the Worlds).

Although the dust was from the Wood Between the Worlds, some of it, he discovered, always wanted to get back to the Wood, and some of it always wanted to get away. His early experiments with the dust had involved him using guinea pigs as test subjects, which resulted in the unfortunate creatures either disappearing forever, or even exploding right on the spot.

From the results of these tests, Andrew was able to create the rings, where the yellow ones would take you to the Wood, and the green ones would take you out of it.

In order to travel with the rings (either yellow or green), one had to be either touching the ring or touching someone who was touching it. For the green rings to work, the wearer also had to jump into one of the many pools in the Wood (see Wood between the Worlds for more information).

Experimentation
After he had first created the rings, Andrew spent a while experimenting with them, by tying the yellow rings to guinea pigs. This resulted, to the delight of Andrew, in several guinea pigs disappearing entirely to 'another world', though of course they had no means of getting back, or telling him about it. Therefore, in order to test for sure whether it would be safe for human travel, he tricked Polly Plummer into touching one of the yellow rings.

Shortly afterwards, Digory did the same in order to rescue Polly, bringing with him two green rings. He found himself transported to the Wood between the Worlds, where he found Polly and the various guinea pigs.

Before returning, the two used the green rings to enter and explore the dying world of Charn, where Digory awakened Jadis, the last empress of Charn, from a centuries-old spell.

Upon leaving with the yellow rings, and returning to their own world with the green ones, they inadvertently brought Jadis back with them, when she grabbed onto Polly's hair. This proved that one did not have to possess the ring for it to function, but, rather, one could simply touch another person who possessed a ring.

Upon arriving on Earth, Jadis immediately began to wreak havoc throughout London, with Andrew Ketterley at her tail. In an effort to return her to her own world, Digory, Polly, Jadis, Uncle Andrew, Frank the Cabby and his horse Strawberry ended up in Narnia to witness it's creation. At the end of their adventure, Digory was commanded by Aslan to bury the rings so that no one could use them again.

Digory did as he was told, by burying the core of the apple from the Silver apple tree in his back garden, and then burying the rings all around it.

In 1949, the Seven Friends of Narnia gathered together and conspired to retrieve the rings.

Before Eustace and Jill returned to Narnia, Peter and Edmund sneaked into the garden of Professor Kirke's old house, and dug up the rings. However, before they could use them, the remainder of the seven friends present were summoned to Aslan's Country, when the train they were travelling on crashed.

What became of the rings in the wreckage after their deaths remains a complete mystery.

Theories
It is possible that the rings are attracted to strong magic, which explains why Digory and Polly are brought to the ruined palace of Charn, where Jadis was awaiting rescue, and soon afterward when they were brought directly to the house in London, where Uncle Andrew was conducting his experiments.