1. The problem statement, all variables and given/known data A piece of metal turns red when at 1100K while a piece of quartz doesn't "shine" (i.e. emits in the visible range I guess) at this temperature. Explain this phenomenon knowing that quartz is transparent to visible light. 2. Relevant equations None. 3. The attempt at a solution This question blows my mind. A first reaction would be to say that the quartz doesn't behaves like a black body since it doesn't absorbs (in the sense of a black body) visible light. This is however totally false in the sense that ice is also transparent and has an emissivity of 0.97 which is almost a black body (in fact I guess it absorbs any other light with different frequencies than light in the visible spectra). Also I don't know the theory of transparent materials. I know they absorbs light and then re emit it with the same wavelength and a perfectly transparent wouldn't absorb any photon (so a white body?), however it wouldn't reflect either, or few. I'm just clueless. If you have any idea, feel free to share.