What's better for the environment, silk or polyester or cotton? It's not a simple answer, since everything comes with different amounts of energy, water, chemicals, and waste. Over eight years, Nike's been cataloging materials in their sustainability index, and they've just released a new app to share what they've learned with other designers. There are more detailed life-assessment tools out there, but this one's easy to use—and free. Right now, it includes info for 50 materials.