I designed this pendant to hold a 3mm x 22.5mm Tritium vial. These vials continuously emit a glow and do NOT need to be recharged by sunlight. They can be purchased from a few places on the Internet. You can read more about Tritium HERE.
Although not bright enough to read a book by they are certainly bright enough to find a bunch of keys in a darkened room or tent, for example.
Available from my shop on Shapeways, direct link HERE, starting at $5, and available in a variety of materials and colours. The tritium vial is NOT included. I also have one green pendant shown here, INCLUDING vial and basic leather necklace, that you can purchase directly from me for $25 plus $5 shipping to US or Canada. The vial simply slips into the pendant from the top and the necklace stops it from coming out.