Actual lost "mystical treasures"
In the real world, the Amulet of Penultimate Truth, the Chalice of Light, the Crown of Life, the Philosopher's Stone, and the Sword of Ultimate Sorcery actually existed. The Amulet of Penultimate Truth was destroyed, and the Chalice of Light is locked away in a vault, but the Crown, Stone, and Sword vanished and nobody knows where they are.
This is true.
They were made by Atari for a series of contests; all worth $25,000, except the sword was worth $50,000. They were going to put out a series of four games, each of which included a secret message. The first few people to discover the message would compete against each other for the Amulet, Chalice, Crown, and Stone respectively; the four winners would then compete for the Sword. However, the company collapsed before the third and fourth games were released, or even completed, and the final three contests were never held.
The winner of the Amulet had it melted down and the gems and gold sold; the winner of the Chalice has had it locked in a safety-deposit box since the Eighties.
However, those final three prizes simply disappeared, and have never been found.