"Sun-colored amethyst globes were originally clear. Glass is naturally a green color. It must have a chemical added to bleach out the green in order to have clear glass. Before World War I, manganese was a chemical often used to achieve clear glass. Exposure to the ultraviolet rays from the sun causes the manganese bleached glass to turn purple. The shade of purple depends on the amount of manganese in the glass and the time of exposure in the sun. Most of the manganese was imported from Germany, and World War I interrupted this."