Legends and fairy tales; like sacred text; are great sources of the bits and pieces of the alchemical secrets. The spiral horn of the unicorn; the fire breathing dragons who live in caves. The sword in the stone ; the (spiral) horn of the cornucopia or horn of plenty. These are all part of the alchemical and astrological lore of the ancient heavenly connections to the Earthly kingdoms. The original zodiacal depictions were about the fertility connections to the terrestrial things. Knowledge is power, and knowledge of fertility is alchemical. Having knowledge of the fertility or reproductive cycles of the plants, animals, minerals is being in possession of the keys to the kingdom of the Earth and everything on it.
If a story mentions the number 12, it is possibly a reference to the 12 constellations. If it mentions the number 7 it is somehow connected to the colors of the visible spectrum. The first quarter of the Moon cycle; is the real, seven days, of creation. So the number 7 isn’t always about the spectrum colors; but it is about creation. If light is involved it has a connection to the creation. If immortality is mentioned; there is a connection to alchemy and gold. As in; for example the story of the garden of Eden.

The story about the sword in the stone is about the spectral spear of a convergent lens; being used or focused on a stone; more specifically a crystalline mass. The dragon living in the earthly caves is set to action by the spectral lens being focused on and into the stone or crystalline mass.

A convergent lens or a parabolic mirror can be used to burn letters of an alphabetical script; into a tablet of stone. The process brings the fiery dragon to life; as the tablets of stone are inscribed. If you went up on top of a hill or mountain; no one down below could see you when you wrote on the tablets. Or, if you had a walled or fenced area to work in; no one could see you while you created the tablets of scripted stones; even though they might see the smoke or flashes of light above the open topped enclosure or tent, or over and above the mountain top; during your operations of the art.