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 granted Jadis immortality and inexhaustible strength, while another apple, which Digory took and brought back to Aslan, flourished into the extremely powerful Tree of Protection, which acted as an invincible shield, guarding Narnia against all evil.
 * 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; the ethereal, breathtaking scent of its magical fruits repelled all enemies 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 would sometimes move by itself mysteriously when there was no wind blowing, and the very sap of the tree never forgot where it came from. It was later proven that there was still magic in its wood, for it was later chopped down and fashioned into the very wooden wardrobe that played a major role in The Lion, the Witch, and the Wardrobe, leading to all the comings and goings between Narnia and Earth.
 * The Tree of Knowledge: discovered thriving in Aslan's Country

The Apples
The fruit of these trees were much like ordinary apples, only they were silvery in appearance and incredibly beautiful, even casting a light of their own beneath the branches of the Tree, and gave off an ethereal, breathtaking, almost irresistible smell. The juice of the apples, however, was a darker colour than one would expect, and stained one’s mouth.

The first person to eat them was Jadis, who dismissed the warning that the apples should only be plucked to help others and not to be eaten for oneself, climbed into the Garden of Youth over its walls, and plucked one solely for herself. After she had greedily ate the fruit, Jadis told Digory Kirke (who had entered the Garden properly: through its golden gates), who had plucked an apple in obedience to Aslan, what the fruit really was, and that she felt such changes in herself that she knew she would never grow old or die. She tried twice to tempt Digory into disobeying Aslan: first, by encouraging him to eat the fruit (which would give endless youth and life) himself, and second, to give the fruit to his dying mother, Mabel Kirke, instead, assuring him that it would cure her of her chronic illness. Digory, fortunately for him and his mother, was able to resist both temptations and even rebuffed Jadis rudely, who retorted by calling him a fool to throw away his one and only chance of endless youth.

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 its brightness threw strange lights on the ceiling. Also, the smell of the apple was so glorious that it was like "a window that opened on Heaven". Digory fed the apple to his mother, who was soon miraculously cured of her illness, just like Jadis and Aslan had promised. 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 had wonderful, powerful magical properties, having different effects on people, depending solely on whether they ate it rightfully and where they were when they ate it. In Narnia, one who ate it would never grow old or ill or die; if eaten illegally, however, they would find that they would never be happy, 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 after she wrongfully ate one of the apples: the rest of the fruit became so dreadful to her that she, according Aslan, would never dare to come within a hundred miles of the Tree of Protection, for its very smell was "death and horror and despair to her". She had, in Aslan's words, "unwearying strength and endless days like a Goddess", and would become increasingly strong in dark Magic, but, just like Aslan had said, everlasting life with an evil heart meant everlasting misery, and that she herself had started to know it - "All get what they want; they do not always like it". Aslan also revealed that, while the fruit always worked according to its nature, it would not work happily for anyone who plucked it at their own will: Narnia itself would have become another strong, cruel empire like Charn, instead of the blissful paradise that Aslan wanted it to be, if the apple that had been planted to protect the country had been a stolen apple. If Digory had succumbed into the second temptation and used the fruit to heal his mother, it would bring neither of them joy, and that the day will come when both of them would have looked back and actually said that it would have been better to die in that illness.

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

Because of the apple she ate, Jadis was able to outlive the Tree of Protection, and returned to Narnia, seizing it as her own and plunging it into 100 years of constant winter and no Christmas. Her cold, cruel ways and evil reign were finally stopped by Aslan and the four Pevensie siblings in The Lion, the Witch, and the Wardrobe.