Finally had a chance to ask that. The lens is covering the heat seeking sensor, which depends on infrared light. Regular glass would not be transparent enough to infrared. Pure silicon would be more transparent, but would block other wavelengths. So if they are dark like these, they are either gallium arsenide or zinc selenide. Both are valuable and can be recycled either as lenses or raw material, but of course after it hits the target, there isn't much.

Finally had a chance to ask that. The lens is covering the heat seeking sensor, which depends on infrared light. Regular glass would not be transparent enough to infrared. Pure silicon would be more transparent, but would block other wavelengths. So if they are dark like these, they are either gallinium arsenide or zinc selenide. Both are valuable and can be recycled either as lenses or raw material, but of course after it hits the target, there isn't much.