The most famous is the Philosopher's Stone: a substance that can convert base metals into gold.
But in itself, that was not the point. It would also transform the owner or user -- it was a hermetic symbol, a mechanical means to "pierce the veil" and to see the deep mystical and magical truths, the Real Reality. It was immanent, a thing in the world, that enabled the transcendent, to go beyond, above, outside of the world. Its discovery would have been the single most important moment in the history of the world, the moment in which humans had a reliable road to divinity.
Hmm. Sounds familiar, doesn't it?
But out of alchemy came modern chemistry, and also some parts of the scientific method. After all, as some smart people worked out, you could systematically try all the permutations of materials that your reading had suggested as possibilities. That meant measuring, weighing, mixing properly, keeping detailed notes. Fundamental lab work is the unglamorous slab of concrete beneath the shining houses of the physical sciences. There were waves of hysteria and hype, but after each, something useful would be left behind, minus the sheen of unlimited dreams.
Hmm. Sounds familiar, doesn't it?
These days it is possible for a device to transmute base metals into gold. But the operators have not, so far as I can deduce, ascended to any higher planes of existence. They have eschewed the ethereal and remained reliably corporeal.