Silver Apple Tree

The silver apple tree was a magic-based species of tree that originated in the Garden of Youth in the western Kingdom of Narnia as early as the dawn of that world.

Known trees
In all existence, there are only four known individual silver apple trees:
 * The Tree of Youth: discovered in the Garden of Youth in Narnian-year 1 by Digory Kirke and Jadis; it was the home to a roosting phoenix who guarded it; one of its apples allowed Jadis to live 1000 years while Digory took another one of the apples to bring back to Aslan
 * The Tree of Protection: planted on the bank of the Great River of Narnia from the apple Digory had taken from the Tree of Youth; its magical fruits repelled Jadis from Narnia for 900 years until it presumably died; Digory took one of its apples back to Earth (with Aslan's approval) to feed to and heal his ailing mother
 * The "Wardrobe Tree": planted on Earth from the core of the apple Digory had fed his mother, it grew in the backyard of Digory Kirke's House, though did not retain all of its original, Narnian magical properties; it was believed to have sometimes moved of its own accord when the wind was not blowing; it was later chopped down and fashioned into the wooden wardrobe that rested inside the house
 * The Tree of Knowledge: discovered thriving in Aslan's Country

The Apples
The fruit of these trees were much like apples, only they were silvery in appearance and beautiful, even casting a light of their own beneath the branches of the Tree. The juice of the apples was a darker colour than one would expect, and stained one’s mouth.

The first person to eat them was Jadis, who climbed over the wall of the Garden of Youth (upon which a sign bore a warning that the apples should only be plucked to help others&mdash;not to be eaten for oneself). Jadis claimed, upon greedily eating one, that she felt these apples had made changes within her so that she knew she would never grow old or die. She approached Digory Kirke (who had entered the Garden properly: through its golden gates) who had plucked an apple in obedience to Aslan. The smell of the apples were often perceived as being almost irresistable, tempting Digory to eat one even when he knew he should not. When Digory returned to Earth (with Aslan's approval) bearing one of the apples, it was described as being so beautiful that in comparison all other colours faded, and the light from the apple was so glourious it was like a "window into Heaven". On Earth, Digory fed the apple to his dying mother, Mabel Kirke, and she was soon cured of her chronic illness. When he planted the core in his backyard, it immediately began to bud, though still did not grow as fast as the Tree of Protection (the immediate offspring of the Tree of Youth) had grown, when planted in Narnia.

The apples were magic, and had different effects on people depending on whether they ate it rightfully and where they were when they ate it. In Narnia, one who ate it would live free of illness and the process of aging so, potentially, one could live forever; if eaten illegally, however, they would find their length of days to be merely length of misery, and the smell and sight of the apples would instill in them a terrible dread that would repel them away from the apples forevermore. Such was the fate of Jadis when she wrongfully ate the apples which became so dreadful to her after she ate the one that she did not dare come within one hundred miles of the Tree of Protection while it stood for nine hundred years in the centre of Narnia. Because it had increased her longevity, however, Jadis actually outlived the Tree of Protection when it died in year 900, allowing her to return to the Kingdom of Narnia to seize the country as her own.

In our world, the apples did not grant eternal or near-eternal life, but they did magically heal as was evident by Mabel Kirke's miraculous recovery.