Gypsum is also used in frequency doubling crystals, which makes sense, because to double the frequency you half the wavelength, and short wavelength Infra-red isn't absorbed by the atmosphere, it goes straight out into space. It's also the principle heating element of sunlight, as it's absorbed by objects, and re-radiated as longer wavelength IR, which is absorbed by air [and greenhouse gasses.]

As you say, it's a very impressive trick. Now we need a few billion square miles of the stuff.