This is said to be an excerpt from the alchemical writings of Glauber and attributed to a work of alchemical gemstones.
recipe from Glauber’s writings (part 1)
Adjoin to the red salt, made of wood or carbon, a little of the powder of white flints. Put them into a crucible, and melt them in a fire that they may become a red glass, resembling almost the colour of a ruby. By continuing the fusion the red colour changes to green, and has the likeness of an emerald. After this comes a sky colour, resembling a sapphire. Then follows a yellow, not unlike a jacynth. Then, after a long continued heat, it becomes black, and like to an agate.
The purifying fire of alchemy book #16 kindle edition