A German-based company, KANN, has manufactured some pretty cool glow in the dark concrete stones. NighTec Leuchtsteine light stones need neither batteries nor solar cells. Light crystals contained in the concrete store energy during the day and emit it at night—all without complicated technology! The light crystals contained in the concrete recharge in normal daylight and emit visible light energy for about 10 hours. The stones remain a white color during the day and turn a dark blue or yellow/green color at night. The crystals are non-toxic and free from harmful radiation. If you’d like more information, although only in German, visit www.kann.de.