For use in indoor or outdoor natural gas fireplaces, eco-friendly FireCrystals have the lowest fine-particulate emissions per unit of heat delivered, the maker says. As a direct replacement for gas or wood logs, FireCrystals emit all the beauty of a burning fire without releasing soot or ash into the environment. Their gemstone look is available in a multitude of colors that add extra interest to the application. 866.875.6615. www.firecrystals.com.