Around 1602, a shoemaker and part-time alchemist from Bologna picked up a mineral in the hills outside the city. While he failed to produce gold with it, he found to his surprise that the stones after being treated and then exposed to sunlight, would glow in the dark like burning coals. The fame of his “sponge of light” spread across Europe, attracting the attention of Galileo and many others, and becoming a sought-after marvel of chymistry. Yet even though detailed recipes for preparing it were published several times, no one outside of Bologna could get the process to work. Thus by the 1660s, the method was considered a “lost secret.” In the 1680s however, Wilhelm Homberg, a young chymist fascinated by the interaction of light with matter, learned how to prepare it better than anyone had done before. This lecture recounts the history of the “Bologna Stone,” explores why so many practitioners failed to get the process to work, and presents my work in reproducing Homberg’s process, discovering along the way a key reason for the seventeenth-century failures.